This series is a follow-up to [1], which adds mTHP support to khugepaged.
mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
configs to make sense. Without it global="defer" and  mTHP="inherit" case
is "undefined" behavior.

We've seen cases were customers switching from RHEL7 to RHEL8 see a
significant increase in the memory footprint for the same workloads.

Through our investigations we found that a large contributing factor to
the increase in RSS was an increase in THP usage.

For workloads like MySQL, or when using allocators like jemalloc, it is
often recommended to set /transparent_hugepages/enabled=never. This is
in part due to performance degradations and increased memory waste.

This series introduces enabled=defer, this setting acts as a middle
ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
page fault handler will act normally, making a hugepage if possible. If
the allocation is not MADV_HUGEPAGE, then the page fault handler will
default to the base size allocation. The caveat is that khugepaged can
still operate on pages that are not MADV_HUGEPAGE.

This allows for three things... one, applications specifically designed to
use hugepages will get them, and two, applications that don't use
hugepages can still benefit from them without aggressively inserting
THPs at every possible chance. This curbs the memory waste, and defers
the use of hugepages to khugepaged. Khugepaged can then scan the memory
for eligible collapsing. Lastly there is the added benefit for those who
want THPs but experience higher latency PFs. Now you can get base page
performance at the PF handler and Hugepage performance for those mappings
after they collapse.

Admins may want to lower max_ptes_none, if not, khugepaged may
aggressively collapse single allocations into hugepages.

TESTING:
- Built for x86_64, aarch64, ppc64le, and s390x
- selftests mm
- In [1] I provided a script [2] that has multiple access patterns
- lots of general use.
- redis testing. This test was my original case for the defer mode. What I
   was able to prove was that THP=always leads to increased max_latency
   cases; hence why it is recommended to disable THPs for redis servers.
   However with 'defer' we dont have the max_latency spikes and can still
   get the system to utilize THPs. I further tested this with the mTHP
   defer setting and found that redis (and probably other jmalloc users)
   can utilize THPs via defer (+mTHP defer) without a large latency
   penalty and some potential gains. I uploaded some mmtest results
   here[3] which compares:
       stock+thp=never
       stock+(m)thp=always
       khugepaged-mthp + defer (max_ptes_none=64)

  The results show that (m)THPs can cause some throughput regression in
  some cases, but also has gains in other cases. The mTHP+defer results
  have more gains and less losses over the (m)THP=always case.

V6 Changes:
- nits
- rebased dependent series and added review tags

V5 Changes:
- rebased dependent series
- added reviewed-by tag on 2/4

V4 Changes:
- Minor Documentation fixes
- rebased the dependent series [1] onto mm-unstable
    commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")

V3 Changes:
- Combined the documentation commits into one, and moved a section to the
  khugepaged mthp patchset

V2 Changes:
- base changes on mTHP khugepaged support
- Fix selftests parsing issue
- add mTHP defer option
- add mTHP defer Documentation

[1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
[2] - https://gitlab.com/npache/khugepaged_mthp_test
[3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html

Nico Pache (4):
  mm: defer THP insertion to khugepaged
  mm: document (m)THP defer usage
  khugepaged: add defer option to mTHP options
  selftests: mm: add defer to thp setting parser

 Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
 include/linux/huge_mm.h                    | 18 +++++-
 mm/huge_memory.c                           | 69 +++++++++++++++++++---
 mm/khugepaged.c                            |  8 +--
 tools/testing/selftests/mm/thp_settings.c  |  1 +
 tools/testing/selftests/mm/thp_settings.h  |  1 +
 6 files changed, 106 insertions(+), 22 deletions(-)

-- 
2.49.0

On Wed, May 14, 2025 at 09:38:53PM -0600, Nico Pache wrote:
> This series is a follow-up to [1], which adds mTHP support to khugepaged.
> mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> configs to make sense. Without it global="defer" and  mTHP="inherit" case
> is "undefined" behavior.

How can this be a follow up to an unmerged series? I'm confused by that.

And you're saying that you're introducing 'undefined behaviour' on the
assumption that another series which seems to have quite a bit of
discussion let to run will be merged?

While I'd understand if this was an RFC just to put the idea out there,
you're not proposing it as such?

Unless there's a really good reason we're doing this way (I may be missing
something), can we just have this as an RFC until the series it depends on
is settled?

>
> We've seen cases were customers switching from RHEL7 to RHEL8 see a
> significant increase in the memory footprint for the same workloads.
>
> Through our investigations we found that a large contributing factor to
> the increase in RSS was an increase in THP usage.
>
> For workloads like MySQL, or when using allocators like jemalloc, it is
> often recommended to set /transparent_hugepages/enabled=never. This is
> in part due to performance degradations and increased memory waste.
>
> This series introduces enabled=defer, this setting acts as a middle
> ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> page fault handler will act normally, making a hugepage if possible. If
> the allocation is not MADV_HUGEPAGE, then the page fault handler will
> default to the base size allocation. The caveat is that khugepaged can
> still operate on pages that are not MADV_HUGEPAGE.
>
> This allows for three things... one, applications specifically designed to
> use hugepages will get them, and two, applications that don't use
> hugepages can still benefit from them without aggressively inserting
> THPs at every possible chance. This curbs the memory waste, and defers
> the use of hugepages to khugepaged. Khugepaged can then scan the memory
> for eligible collapsing. Lastly there is the added benefit for those who
> want THPs but experience higher latency PFs. Now you can get base page
> performance at the PF handler and Hugepage performance for those mappings
> after they collapse.
>
> Admins may want to lower max_ptes_none, if not, khugepaged may
> aggressively collapse single allocations into hugepages.
>
> TESTING:
> - Built for x86_64, aarch64, ppc64le, and s390x
> - selftests mm
> - In [1] I provided a script [2] that has multiple access patterns
> - lots of general use.

OK so this truly is dependent on the unmerged series? Or isn't it?

Is your testing based on that?

Because again... that surely makes this series a no-go until we land the
prior (which might be changed, and thus necessitate re-testing).

Are you going to provide any of these numbers/data anywhere?

> - redis testing. This test was my original case for the defer mode. What I
>    was able to prove was that THP=always leads to increased max_latency
>    cases; hence why it is recommended to disable THPs for redis servers.
>    However with 'defer' we dont have the max_latency spikes and can still
>    get the system to utilize THPs. I further tested this with the mTHP
>    defer setting and found that redis (and probably other jmalloc users)
>    can utilize THPs via defer (+mTHP defer) without a large latency
>    penalty and some potential gains. I uploaded some mmtest results
>    here[3] which compares:
>        stock+thp=never
>        stock+(m)thp=always
>        khugepaged-mthp + defer (max_ptes_none=64)
>
>   The results show that (m)THPs can cause some throughput regression in
>   some cases, but also has gains in other cases. The mTHP+defer results
>   have more gains and less losses over the (m)THP=always case.
>
> V6 Changes:
> - nits
> - rebased dependent series and added review tags
>
> V5 Changes:
> - rebased dependent series
> - added reviewed-by tag on 2/4
>
> V4 Changes:
> - Minor Documentation fixes
> - rebased the dependent series [1] onto mm-unstable
>     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
>
> V3 Changes:
> - Combined the documentation commits into one, and moved a section to the
>   khugepaged mthp patchset
>
> V2 Changes:
> - base changes on mTHP khugepaged support
> - Fix selftests parsing issue
> - add mTHP defer option
> - add mTHP defer Documentation
>
> [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> [2] - https://gitlab.com/npache/khugepaged_mthp_test
> [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
>
> Nico Pache (4):
>   mm: defer THP insertion to khugepaged
>   mm: document (m)THP defer usage
>   khugepaged: add defer option to mTHP options
>   selftests: mm: add defer to thp setting parser
>
>  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
>  include/linux/huge_mm.h                    | 18 +++++-
>  mm/huge_memory.c                           | 69 +++++++++++++++++++---
>  mm/khugepaged.c                            |  8 +--
>  tools/testing/selftests/mm/thp_settings.c  |  1 +
>  tools/testing/selftests/mm/thp_settings.h  |  1 +
>  6 files changed, 106 insertions(+), 22 deletions(-)
>
> --
> 2.49.0
>

On Tue, May 20, 2025 at 3:43 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Wed, May 14, 2025 at 09:38:53PM -0600, Nico Pache wrote:
> > This series is a follow-up to [1], which adds mTHP support to khugepaged.
> > mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> > configs to make sense. Without it global="defer" and  mTHP="inherit" case
> > is "undefined" behavior.
>
> How can this be a follow up to an unmerged series? I'm confused by that.
Hi Lorenzo,

follow up or loose dependency. Not sure the correct terminology.

Either way, as I was developing this as a potential solution for the
THP internal fragmentation issue, upstream was working on adding
mTHPs. By adding a new THP sysctl entry I noticed mTHP would now be
missing the same entry. Furthermore I was told mTHP support for
khugepaged was a desire, so I began working on it in conjunction. So
given the undefined behavior of defer globally while any mix of mTHP
settings, it became dependent on the khugepaged support. Either way
patch 1 of this series is the core functionality. The rest is to fill
the undefined behavior gap.
>
> And you're saying that you're introducing 'undefined behaviour' on the
> assumption that another series which seems to have quite a bit of
> discussion let to run will be merged?
This could technically get merged without the mTHP khugepaged changes,
but then the reviews would probably all be pointing out what I pointed
out above. Chicken or Egg problem...
>
> While I'd understand if this was an RFC just to put the idea out there,
> you're not proposing it as such?
Nope we've already discussed this in both the MM alignment and thp
upstream meetings, no one was opposing it, and a lot of testing was
done-- by me, RH's CI, and our perf teams. Ive posted several RFCs
before posting a patchset.
>
> Unless there's a really good reason we're doing this way (I may be missing
> something), can we just have this as an RFC until the series it depends on
> is settled?
Hopefully paragraph one clears this up! They were built in
conjunction, but posting them as one series didn't feel right (and
IIRC this was also discussed, and this was decided).
>
> >
> > We've seen cases were customers switching from RHEL7 to RHEL8 see a
> > significant increase in the memory footprint for the same workloads.
> >
> > Through our investigations we found that a large contributing factor to
> > the increase in RSS was an increase in THP usage.
> >
> > For workloads like MySQL, or when using allocators like jemalloc, it is
> > often recommended to set /transparent_hugepages/enabled=never. This is
> > in part due to performance degradations and increased memory waste.
> >
> > This series introduces enabled=defer, this setting acts as a middle
> > ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> > page fault handler will act normally, making a hugepage if possible. If
> > the allocation is not MADV_HUGEPAGE, then the page fault handler will
> > default to the base size allocation. The caveat is that khugepaged can
> > still operate on pages that are not MADV_HUGEPAGE.
> >
> > This allows for three things... one, applications specifically designed to
> > use hugepages will get them, and two, applications that don't use
> > hugepages can still benefit from them without aggressively inserting
> > THPs at every possible chance. This curbs the memory waste, and defers
> > the use of hugepages to khugepaged. Khugepaged can then scan the memory
> > for eligible collapsing. Lastly there is the added benefit for those who
> > want THPs but experience higher latency PFs. Now you can get base page
> > performance at the PF handler and Hugepage performance for those mappings
> > after they collapse.
> >
> > Admins may want to lower max_ptes_none, if not, khugepaged may
> > aggressively collapse single allocations into hugepages.
> >
> > TESTING:
> > - Built for x86_64, aarch64, ppc64le, and s390x
> > - selftests mm
> > - In [1] I provided a script [2] that has multiple access patterns
> > - lots of general use.
>
> OK so this truly is dependent on the unmerged series? Or isn't it?
>
> Is your testing based on that?
Most of the testing was done in conjunction, but independent testing
was also done on this series (including by a large customer that was
itching to try the changes, and they were very satisfied with the
results).
>
> Because again... that surely makes this series a no-go until we land the
> prior (which might be changed, and thus necessitate re-testing).
>
> Are you going to provide any of these numbers/data anywhere?
There is a link to the results in this cover letter
[3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
>
> > - redis testing. This test was my original case for the defer mode. What I
> >    was able to prove was that THP=always leads to increased max_latency
> >    cases; hence why it is recommended to disable THPs for redis servers.
> >    However with 'defer' we dont have the max_latency spikes and can still
> >    get the system to utilize THPs. I further tested this with the mTHP
> >    defer setting and found that redis (and probably other jmalloc users)
> >    can utilize THPs via defer (+mTHP defer) without a large latency
> >    penalty and some potential gains. I uploaded some mmtest results
> >    here[3] which compares:
> >        stock+thp=never
> >        stock+(m)thp=always
> >        khugepaged-mthp + defer (max_ptes_none=64)
> >
> >   The results show that (m)THPs can cause some throughput regression in
> >   some cases, but also has gains in other cases. The mTHP+defer results
> >   have more gains and less losses over the (m)THP=always case.
> >
> > V6 Changes:
> > - nits
> > - rebased dependent series and added review tags
> >
> > V5 Changes:
> > - rebased dependent series
> > - added reviewed-by tag on 2/4
> >
> > V4 Changes:
> > - Minor Documentation fixes
> > - rebased the dependent series [1] onto mm-unstable
> >     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
> >
> > V3 Changes:
> > - Combined the documentation commits into one, and moved a section to the
> >   khugepaged mthp patchset
> >
> > V2 Changes:
> > - base changes on mTHP khugepaged support
> > - Fix selftests parsing issue
> > - add mTHP defer option
> > - add mTHP defer Documentation
> >
> > [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> > [2] - https://gitlab.com/npache/khugepaged_mthp_test
> > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> >
> > Nico Pache (4):
> >   mm: defer THP insertion to khugepaged
> >   mm: document (m)THP defer usage
> >   khugepaged: add defer option to mTHP options
> >   selftests: mm: add defer to thp setting parser
> >
> >  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
> >  include/linux/huge_mm.h                    | 18 +++++-
> >  mm/huge_memory.c                           | 69 +++++++++++++++++++---
> >  mm/khugepaged.c                            |  8 +--
> >  tools/testing/selftests/mm/thp_settings.c  |  1 +
> >  tools/testing/selftests/mm/thp_settings.h  |  1 +
> >  6 files changed, 106 insertions(+), 22 deletions(-)
> >
> > --
> > 2.49.0
> >
>

To start with I do apologise for coming to this at v6, I realise it's
irritating to have push back at this late stage. This is more so my attempt
to understand where this series -sits- so I can properly review it.

So please bear with me here :)

So, I remain very confused. This may just be a _me_ thing here :)

So let me check my understanding:

1. This series introduces this new THP deferred mode.
2. By 'follow-up' really you mean 'inspired by' or 'related to' right?
3. If this series lands before [1], commits 2 - 4 are 'undefined
   behaviour'.

In my view if 3 is true this series should be RFC until [1] merges.

If I've got it wrong and this needs to land first, we should RFC [1].

That way we can un-RFC once the dependency is met.

We have about 5 [m]THP series in flight at the moment, all touching at
least vaguely related stuff, so any help for reviewers would be hugely
appreciated thanks :)

On Wed, May 21, 2025 at 04:41:54AM -0600, Nico Pache wrote:
> On Tue, May 20, 2025 at 3:43 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > On Wed, May 14, 2025 at 09:38:53PM -0600, Nico Pache wrote:
> > > This series is a follow-up to [1], which adds mTHP support to khugepaged.
> > > mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> > > configs to make sense. Without it global="defer" and  mTHP="inherit" case
> > > is "undefined" behavior.
> >
> > How can this be a follow up to an unmerged series? I'm confused by that.
> Hi Lorenzo,
>
> follow up or loose dependency. Not sure the correct terminology.
>

See above. Let's nail this down please.

> Either way, as I was developing this as a potential solution for the
> THP internal fragmentation issue, upstream was working on adding
> mTHPs. By adding a new THP sysctl entry I noticed mTHP would now be
> missing the same entry. Furthermore I was told mTHP support for
> khugepaged was a desire, so I began working on it in conjunction. So
> given the undefined behavior of defer globally while any mix of mTHP
> settings, it became dependent on the khugepaged support. Either way
> patch 1 of this series is the core functionality. The rest is to fill
> the undefined behavior gap.
> >
> > And you're saying that you're introducing 'undefined behaviour' on the
> > assumption that another series which seems to have quite a bit of
> > discussion let to run will be merged?
> This could technically get merged without the mTHP khugepaged changes,
> but then the reviews would probably all be pointing out what I pointed
> out above. Chicken or Egg problem...
> >
> > While I'd understand if this was an RFC just to put the idea out there,
> > you're not proposing it as such?
> Nope we've already discussed this in both the MM alignment and thp
> upstream meetings, no one was opposing it, and a lot of testing was
> done-- by me, RH's CI, and our perf teams. Ive posted several RFCs
> before posting a patchset.
> >
> > Unless there's a really good reason we're doing this way (I may be missing
> > something), can we just have this as an RFC until the series it depends on
> > is settled?
> Hopefully paragraph one clears this up! They were built in
> conjunction, but posting them as one series didn't feel right (and
> IIRC this was also discussed, and this was decided).

'This was also discussed and this was decided' :)

I'm guessing rather you mean discussion was had with other reviewers and of
course our earstwhile THP maintainer David, and you guys decided this made
more sense?

Obviously upstream discussion is what counts, but as annoying as it is, one
does have to address the concerns of reviewers even if late to a series
(again, apologies for this).

So, to be clear - I'm not intending to hold up or block the series, I just
want to understand how things are, this is the purpose here.

Thanks!

> >
> > >
> > > We've seen cases were customers switching from RHEL7 to RHEL8 see a
> > > significant increase in the memory footprint for the same workloads.
> > >
> > > Through our investigations we found that a large contributing factor to
> > > the increase in RSS was an increase in THP usage.
> > >
> > > For workloads like MySQL, or when using allocators like jemalloc, it is
> > > often recommended to set /transparent_hugepages/enabled=never. This is
> > > in part due to performance degradations and increased memory waste.
> > >
> > > This series introduces enabled=defer, this setting acts as a middle
> > > ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> > > page fault handler will act normally, making a hugepage if possible. If
> > > the allocation is not MADV_HUGEPAGE, then the page fault handler will
> > > default to the base size allocation. The caveat is that khugepaged can
> > > still operate on pages that are not MADV_HUGEPAGE.
> > >
> > > This allows for three things... one, applications specifically designed to
> > > use hugepages will get them, and two, applications that don't use
> > > hugepages can still benefit from them without aggressively inserting
> > > THPs at every possible chance. This curbs the memory waste, and defers
> > > the use of hugepages to khugepaged. Khugepaged can then scan the memory
> > > for eligible collapsing. Lastly there is the added benefit for those who
> > > want THPs but experience higher latency PFs. Now you can get base page
> > > performance at the PF handler and Hugepage performance for those mappings
> > > after they collapse.
> > >
> > > Admins may want to lower max_ptes_none, if not, khugepaged may
> > > aggressively collapse single allocations into hugepages.
> > >
> > > TESTING:
> > > - Built for x86_64, aarch64, ppc64le, and s390x
> > > - selftests mm
> > > - In [1] I provided a script [2] that has multiple access patterns
> > > - lots of general use.
> >
> > OK so this truly is dependent on the unmerged series? Or isn't it?
> >
> > Is your testing based on that?
> Most of the testing was done in conjunction, but independent testing
> was also done on this series (including by a large customer that was
> itching to try the changes, and they were very satisfied with the
> results).

You should make this very clear in the cover letter.

> >
> > Because again... that surely makes this series a no-go until we land the
> > prior (which might be changed, and thus necessitate re-testing).
> >
> > Are you going to provide any of these numbers/data anywhere?
> There is a link to the results in this cover letter
> [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> >

Ultimately it's not ok in mm to have a link to a website that might go away
any time, these cover letters are 'baked in' to the commit log. Are you
sure this website with 'testoutput2' will exist in 10 years time? :)

You should at the very least add a summary of this data in the cover
letter, perhaps referring back to this link as 'at the time of writing full
results are available at' or something like this.

> > > - redis testing. This test was my original case for the defer mode. What I
> > >    was able to prove was that THP=always leads to increased max_latency
> > >    cases; hence why it is recommended to disable THPs for redis servers.
> > >    However with 'defer' we dont have the max_latency spikes and can still
> > >    get the system to utilize THPs. I further tested this with the mTHP
> > >    defer setting and found that redis (and probably other jmalloc users)
> > >    can utilize THPs via defer (+mTHP defer) without a large latency
> > >    penalty and some potential gains. I uploaded some mmtest results
> > >    here[3] which compares:
> > >        stock+thp=never
> > >        stock+(m)thp=always
> > >        khugepaged-mthp + defer (max_ptes_none=64)
> > >
> > >   The results show that (m)THPs can cause some throughput regression in
> > >   some cases, but also has gains in other cases. The mTHP+defer results
> > >   have more gains and less losses over the (m)THP=always case.
> > >
> > > V6 Changes:
> > > - nits
> > > - rebased dependent series and added review tags
> > >
> > > V5 Changes:
> > > - rebased dependent series
> > > - added reviewed-by tag on 2/4
> > >
> > > V4 Changes:
> > > - Minor Documentation fixes
> > > - rebased the dependent series [1] onto mm-unstable
> > >     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
> > >
> > > V3 Changes:
> > > - Combined the documentation commits into one, and moved a section to the
> > >   khugepaged mthp patchset
> > >
> > > V2 Changes:
> > > - base changes on mTHP khugepaged support
> > > - Fix selftests parsing issue
> > > - add mTHP defer option
> > > - add mTHP defer Documentation
> > >
> > > [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> > > [2] - https://gitlab.com/npache/khugepaged_mthp_test
> > > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> > >
> > > Nico Pache (4):
> > >   mm: defer THP insertion to khugepaged
> > >   mm: document (m)THP defer usage
> > >   khugepaged: add defer option to mTHP options
> > >   selftests: mm: add defer to thp setting parser
> > >
> > >  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
> > >  include/linux/huge_mm.h                    | 18 +++++-
> > >  mm/huge_memory.c                           | 69 +++++++++++++++++++---
> > >  mm/khugepaged.c                            |  8 +--
> > >  tools/testing/selftests/mm/thp_settings.c  |  1 +
> > >  tools/testing/selftests/mm/thp_settings.h  |  1 +
> > >  6 files changed, 106 insertions(+), 22 deletions(-)
> > >
> > > --
> > > 2.49.0
> > >
> >
>

Cheers, Lorenzo

On Wed, May 21, 2025 at 5:25 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> To start with I do apologise for coming to this at v6, I realise it's
> irritating to have push back at this late stage. This is more so my attempt
> to understand where this series -sits- so I can properly review it.

No worries at all! The only thing that frustrates/upsets me in
upstream mailing lists is unprovoked rudeness (which you have not
been).
>
> So please bear with me here :)
>
> So, I remain very confused. This may just be a _me_ thing here :)
>
> So let me check my understanding:
>
> 1. This series introduces this new THP deferred mode.
> 2. By 'follow-up' really you mean 'inspired by' or 'related to' right?
> 3. If this series lands before [1], commits 2 - 4 are 'undefined
>    behaviour'.
The khugepaged mTHP support should land first as without it, adding a
defer option to the global parameters, makes for undefined behavior in
the sysctls from a admin perspective.
>
> In my view if 3 is true this series should be RFC until [1] merges.
Ideally I was trying to get them merged together (Andrew actually had
them both in mm-new a few weeks ago, but a bug was found that got it
pulled, but that is fixed now). The series' complement each other
nicely.
>
> If I've got it wrong and this needs to land first, we should RFC [1].
The khugepaged series [1] should get merged first, but I was shooting
for both at the same time.
>
> That way we can un-RFC once the dependency is met.
>
> We have about 5 [m]THP series in flight at the moment, all touching at
> least vaguely related stuff, so any help for reviewers would be hugely
> appreciated thanks :)
>
> On Wed, May 21, 2025 at 04:41:54AM -0600, Nico Pache wrote:
> > On Tue, May 20, 2025 at 3:43 AM Lorenzo Stoakes
> > <lorenzo.stoakes@oracle.com> wrote:
> > >
> > > On Wed, May 14, 2025 at 09:38:53PM -0600, Nico Pache wrote:
> > > > This series is a follow-up to [1], which adds mTHP support to khugepaged.
> > > > mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> > > > configs to make sense. Without it global="defer" and  mTHP="inherit" case
> > > > is "undefined" behavior.
> > >
> > > How can this be a follow up to an unmerged series? I'm confused by that.
> > Hi Lorenzo,
> >
> > follow up or loose dependency. Not sure the correct terminology.
> >
>
> See above. Let's nail this down please.
>
> > Either way, as I was developing this as a potential solution for the
> > THP internal fragmentation issue, upstream was working on adding
> > mTHPs. By adding a new THP sysctl entry I noticed mTHP would now be
> > missing the same entry. Furthermore I was told mTHP support for
> > khugepaged was a desire, so I began working on it in conjunction. So
> > given the undefined behavior of defer globally while any mix of mTHP
> > settings, it became dependent on the khugepaged support. Either way
> > patch 1 of this series is the core functionality. The rest is to fill
> > the undefined behavior gap.
> > >
> > > And you're saying that you're introducing 'undefined behaviour' on the
> > > assumption that another series which seems to have quite a bit of
> > > discussion let to run will be merged?
> > This could technically get merged without the mTHP khugepaged changes,
> > but then the reviews would probably all be pointing out what I pointed
> > out above. Chicken or Egg problem...
> > >
> > > While I'd understand if this was an RFC just to put the idea out there,
> > > you're not proposing it as such?
> > Nope we've already discussed this in both the MM alignment and thp
> > upstream meetings, no one was opposing it, and a lot of testing was
> > done-- by me, RH's CI, and our perf teams. Ive posted several RFCs
> > before posting a patchset.
> > >
> > > Unless there's a really good reason we're doing this way (I may be missing
> > > something), can we just have this as an RFC until the series it depends on
> > > is settled?
> > Hopefully paragraph one clears this up! They were built in
> > conjunction, but posting them as one series didn't feel right (and
> > IIRC this was also discussed, and this was decided).
>
> 'This was also discussed and this was decided' :)
>
> I'm guessing rather you mean discussion was had with other reviewers and of
> course our earstwhile THP maintainer David, and you guys decided this made
> more sense?
>
> Obviously upstream discussion is what counts, but as annoying as it is, one
> does have to address the concerns of reviewers even if late to a series
> (again, apologies for this).
>
> So, to be clear - I'm not intending to hold up or block the series, I just
> want to understand how things are, this is the purpose here.

Thanks I do appreciate the discussion around the process as I am
fairly new to upstream work (at least to this magnitude). I have been
mostly downstream focused for the last 6 years and I'm trying to shift
upstream as much as possible. So please bear with me as I learn all
the minor undocumented caveats!
>
> Thanks!
>
> > >
> > > >
> > > > We've seen cases were customers switching from RHEL7 to RHEL8 see a
> > > > significant increase in the memory footprint for the same workloads.
> > > >
> > > > Through our investigations we found that a large contributing factor to
> > > > the increase in RSS was an increase in THP usage.
> > > >
> > > > For workloads like MySQL, or when using allocators like jemalloc, it is
> > > > often recommended to set /transparent_hugepages/enabled=never. This is
> > > > in part due to performance degradations and increased memory waste.
> > > >
> > > > This series introduces enabled=defer, this setting acts as a middle
> > > > ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> > > > page fault handler will act normally, making a hugepage if possible. If
> > > > the allocation is not MADV_HUGEPAGE, then the page fault handler will
> > > > default to the base size allocation. The caveat is that khugepaged can
> > > > still operate on pages that are not MADV_HUGEPAGE.
> > > >
> > > > This allows for three things... one, applications specifically designed to
> > > > use hugepages will get them, and two, applications that don't use
> > > > hugepages can still benefit from them without aggressively inserting
> > > > THPs at every possible chance. This curbs the memory waste, and defers
> > > > the use of hugepages to khugepaged. Khugepaged can then scan the memory
> > > > for eligible collapsing. Lastly there is the added benefit for those who
> > > > want THPs but experience higher latency PFs. Now you can get base page
> > > > performance at the PF handler and Hugepage performance for those mappings
> > > > after they collapse.
> > > >
> > > > Admins may want to lower max_ptes_none, if not, khugepaged may
> > > > aggressively collapse single allocations into hugepages.
> > > >
> > > > TESTING:
> > > > - Built for x86_64, aarch64, ppc64le, and s390x
> > > > - selftests mm
> > > > - In [1] I provided a script [2] that has multiple access patterns
> > > > - lots of general use.
> > >
> > > OK so this truly is dependent on the unmerged series? Or isn't it?
> > >
> > > Is your testing based on that?
> > Most of the testing was done in conjunction, but independent testing
> > was also done on this series (including by a large customer that was
> > itching to try the changes, and they were very satisfied with the
> > results).
>
> You should make this very clear in the cover letter.
I will try to do better at updating and providing more information in
my cover letters and patches. I was never sure how much information to
include! I guess the more the merrier.
>
> > >
> > > Because again... that surely makes this series a no-go until we land the
> > > prior (which might be changed, and thus necessitate re-testing).
> > >
> > > Are you going to provide any of these numbers/data anywhere?
> > There is a link to the results in this cover letter
> > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> > >
>
> Ultimately it's not ok in mm to have a link to a website that might go away
> any time, these cover letters are 'baked in' to the commit log. Are you
> sure this website with 'testoutput2' will exist in 10 years time? :)
>
> You should at the very least add a summary of this data in the cover
> letter, perhaps referring back to this link as 'at the time of writing full
> results are available at' or something like this.

Ok good to know I will find a way to summarize the performance and
result changes more cleanly in the cover letter.
>
> > > > - redis testing. This test was my original case for the defer mode. What I
> > > >    was able to prove was that THP=always leads to increased max_latency
> > > >    cases; hence why it is recommended to disable THPs for redis servers.
> > > >    However with 'defer' we dont have the max_latency spikes and can still
> > > >    get the system to utilize THPs. I further tested this with the mTHP
> > > >    defer setting and found that redis (and probably other jmalloc users)
> > > >    can utilize THPs via defer (+mTHP defer) without a large latency
> > > >    penalty and some potential gains. I uploaded some mmtest results
> > > >    here[3] which compares:
> > > >        stock+thp=never
> > > >        stock+(m)thp=always
> > > >        khugepaged-mthp + defer (max_ptes_none=64)
> > > >
> > > >   The results show that (m)THPs can cause some throughput regression in
> > > >   some cases, but also has gains in other cases. The mTHP+defer results
> > > >   have more gains and less losses over the (m)THP=always case.
> > > >
> > > > V6 Changes:
> > > > - nits
> > > > - rebased dependent series and added review tags
> > > >
> > > > V5 Changes:
> > > > - rebased dependent series
> > > > - added reviewed-by tag on 2/4
> > > >
> > > > V4 Changes:
> > > > - Minor Documentation fixes
> > > > - rebased the dependent series [1] onto mm-unstable
> > > >     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
> > > >
> > > > V3 Changes:
> > > > - Combined the documentation commits into one, and moved a section to the
> > > >   khugepaged mthp patchset
> > > >
> > > > V2 Changes:
> > > > - base changes on mTHP khugepaged support
> > > > - Fix selftests parsing issue
> > > > - add mTHP defer option
> > > > - add mTHP defer Documentation
> > > >
> > > > [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> > > > [2] - https://gitlab.com/npache/khugepaged_mthp_test
> > > > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> > > >
> > > > Nico Pache (4):
> > > >   mm: defer THP insertion to khugepaged
> > > >   mm: document (m)THP defer usage
> > > >   khugepaged: add defer option to mTHP options
> > > >   selftests: mm: add defer to thp setting parser
> > > >
> > > >  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
> > > >  include/linux/huge_mm.h                    | 18 +++++-
> > > >  mm/huge_memory.c                           | 69 +++++++++++++++++++---
> > > >  mm/khugepaged.c                            |  8 +--
> > > >  tools/testing/selftests/mm/thp_settings.c  |  1 +
> > > >  tools/testing/selftests/mm/thp_settings.h  |  1 +
> > > >  6 files changed, 106 insertions(+), 22 deletions(-)
> > > >
> > > > --
> > > > 2.49.0
> > > >
> > >
> >
>
> Cheers, Lorenzo
>

On 21.05.25 13:24, Lorenzo Stoakes wrote:
> To start with I do apologise for coming to this at v6, I realise it's
> irritating to have push back at this late stage. This is more so my attempt
> to understand where this series -sits- so I can properly review it.
> 
> So please bear with me here :)
> 
> So, I remain very confused. This may just be a _me_ thing here :)
> 
> So let me check my understanding:
> 
> 1. This series introduces this new THP deferred mode.
> 2. By 'follow-up' really you mean 'inspired by' or 'related to' right?
> 3. If this series lands before [1], commits 2 - 4 are 'undefined
>     behaviour'.
> 
> In my view if 3 is true this series should be RFC until [1] merges.
> 
> If I've got it wrong and this needs to land first, we should RFC [1].
> 
> That way we can un-RFC once the dependency is met.

I really don't have a strong opinion on the RFC vs. !RFC like others 
here -- as long as the dependency is obvious. I treat RFC more as a 
"rough idea" than well tested work.

Anyhow, to me the dependency is obvious, but I've followed the MM 
meeting discussions, development etc.

I interpret "follow up" as "depends on" here. Likely we should have 
spelled out "This series depends on the patch series X that was not 
merged yet, and likely a new version will be required once merged.".

In this particular case, maybe we should just have sent one initial RFC, 
and then rebased it on top of the other work on a public git branch 
(linked from the RFC cover letter).

Once the dependency gets merged, we could just resend the series. 
Looking at the changelog, only minor stuff changed (mostly rebasing etc).

Moving forward, I don't think there is the need to resend as long as the 
dependency isn't merged upstream (or close to being merged upstream) yet.

>>> Because again... that surely makes this series a no-go until we land the
>>> prior (which might be changed, and thus necessitate re-testing).
>>>
>>> Are you going to provide any of these numbers/data anywhere?
>> There is a link to the results in this cover letter
>> [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
>>>
> 
> Ultimately it's not ok in mm to have a link to a website that might go away
> any time, these cover letters are 'baked in' to the commit log. Are you
> sure this website with 'testoutput2' will exist in 10 years time? :)
> 
> You should at the very least add a summary of this data in the cover
> letter, perhaps referring back to this link as 'at the time of writing full
> results are available at' or something like this.

Yeah, or of they were included in some other mail, we can link to that 
mail in lore.

-- 
Cheers,

David / dhildenb

I think the TL;DR here to avoid too much back and forth is - let's please
make this super super simple :)

I would prefer anything that has a dependency to just sit in RFC until the
dependency is merged.

Or, alternatively, to have a big note at the top:

ANDREW - Please do not merge in mm-unstable until series [1] is merged, and
when that is merged please ping for a resend.

Or whatever it might be.

On Wed, May 21, 2025 at 01:46:38PM +0200, David Hildenbrand wrote:
> On 21.05.25 13:24, Lorenzo Stoakes wrote:
> > To start with I do apologise for coming to this at v6, I realise it's
> > irritating to have push back at this late stage. This is more so my attempt
> > to understand where this series -sits- so I can properly review it.
> >
> > So please bear with me here :)
> >
> > So, I remain very confused. This may just be a _me_ thing here :)
> >
> > So let me check my understanding:
> >
> > 1. This series introduces this new THP deferred mode.
> > 2. By 'follow-up' really you mean 'inspired by' or 'related to' right?
> > 3. If this series lands before [1], commits 2 - 4 are 'undefined
> >     behaviour'.
> >
> > In my view if 3 is true this series should be RFC until [1] merges.
> >
> > If I've got it wrong and this needs to land first, we should RFC [1].
> >
> > That way we can un-RFC once the dependency is met.
>
> I really don't have a strong opinion on the RFC vs. !RFC like others here --
> as long as the dependency is obvious. I treat RFC more as a "rough idea"
> than well tested work.
>
> Anyhow, to me the dependency is obvious, but I've followed the MM meeting
> discussions, development etc.

Right but is it clear to Andrew? I mean the cover letter was super unclear
to me.

What's to prevent things getting merged out of order? And do people 'just
have to remember' to resend? And a resend doesn't necessarily mean patch
set X will come after patch set Y.

If there's a requirement related to the ordering of these series it really
has to be expressed very clearly.

(by the way, I feel expressing things like this is a kind of area where we
have _some kind_ of a break down in kernel process or it'd be nice to have
tags or something to properly express this sort of thing. But maybe another
discussion :)

>
> I interpret "follow up" as "depends on" here. Likely we should have spelled
> out "This series depends on the patch series X that was not merged yet, and
> likely a new version will be required once merged.".
>
> In this particular case, maybe we should just have sent one initial RFC, and
> then rebased it on top of the other work on a public git branch (linked from
> the RFC cover letter).
>
> Once the dependency gets merged, we could just resend the series. Looking at
> the changelog, only minor stuff changed (mostly rebasing etc).
>
> Moving forward, I don't think there is the need to resend as long as the
> dependency isn't merged upstream (or close to being merged upstream) yet.

I mean this is still 'just have to remember' stuff :)

Do we need patches 2-4 if the dependency isn't merged? That was unclear to
me.

>
> > > > Because again... that surely makes this series a no-go until we land the
> > > > prior (which might be changed, and thus necessitate re-testing).
> > > >
> > > > Are you going to provide any of these numbers/data anywhere?
> > > There is a link to the results in this cover letter
> > > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> > > >
> >
> > Ultimately it's not ok in mm to have a link to a website that might go away
> > any time, these cover letters are 'baked in' to the commit log. Are you
> > sure this website with 'testoutput2' will exist in 10 years time? :)
> >
> > You should at the very least add a summary of this data in the cover
> > letter, perhaps referring back to this link as 'at the time of writing full
> > results are available at' or something like this.
>
> Yeah, or of they were included in some other mail, we can link to that mail
> in lore.
>
> --
> Cheers,
>
> David / dhildenb
>

>>
>> Anyhow, to me the dependency is obvious, but I've followed the MM meeting
>> discussions, development etc.
> 
> Right but is it clear to Andrew? I mean the cover letter was super unclear
> to me.

I mean, assuming that it would not be clear to Andrew (and I think it is 
clear to Andrew), I we would get CCed on these emails and could 
immediately scream STOOOOOP :)

And until this would hit mm-stable, a bit more time would pass.

> 
> What's to prevent things getting merged out of order?

Fortunately, there are still people working here and not machines (at 
least, that's what I hope).

> And do people 'just
> have to remember' to resend?

Yes, in this case Nico wants to get his stuff upstream and must drive it 
once the dependencies are met IMHO.

> 
> If there's a requirement related to the ordering of these series it really
> has to be expressed very clearly.

Jup. I'll note that for now there was no strict rule what to tag as RFC 
and what not that I know of. Of course, if people send broken, 
half-implemented, untested ... crap, it should *clearly* be RFC.

People should be spelling out dependencies in any case (especially for 
non-RFC versions) clearly.

I'll note that even if there would be a rule, I'm afraid we don't have a 
good place to document it (and not sure if people would find it or even 
try finding it ...) :/

A big problem is when some subsystems have their own rules for how to 
handle such things. That causes major pain for contributors ...

-- 
Cheers,

David / dhildenb

Fundamentally I trust you to make sure this all goes correctly so let's not
belabour the point or delay things here :)

So in that vein, Nico - I would sugesst for future respins adding a really
clear bit to the header as David suggested :) also update the cover letter
tests so it isn't reliant on a possibly ephemeral web link.

But otherwise let's proceed as was.

On Wed, May 21, 2025 at 02:24:45PM +0200, David Hildenbrand wrote:
> > >
> > > Anyhow, to me the dependency is obvious, but I've followed the MM meeting
> > > discussions, development etc.
> >
> > Right but is it clear to Andrew? I mean the cover letter was super unclear
> > to me.
>
> I mean, assuming that it would not be clear to Andrew (and I think it is
> clear to Andrew), I we would get CCed on these emails and could immediately
> scream STOOOOOP :)
>
> And until this would hit mm-stable, a bit more time would pass.
>
> >
> > What's to prevent things getting merged out of order?
>
> Fortunately, there are still people working here and not machines (at least,
> that's what I hope).

Obligatory link to this :P

https://www.youtube.com/watch?v=5lsExRvJTAI

>
> > And do people 'just
> > have to remember' to resend?
>
> Yes, in this case Nico wants to get his stuff upstream and must drive it
> once the dependencies are met IMHO.
>
> >
> > If there's a requirement related to the ordering of these series it really
> > has to be expressed very clearly.
>
> Jup. I'll note that for now there was no strict rule what to tag as RFC and
> what not that I know of. Of course, if people send broken, half-implemented,
> untested ... crap, it should *clearly* be RFC.
>
> People should be spelling out dependencies in any case (especially for
> non-RFC versions) clearly.
>
> I'll note that even if there would be a rule, I'm afraid we don't have a
> good place to document it (and not sure if people would find it or even try
> finding it ...) :/

Yeah... :)

>
> A big problem is when some subsystems have their own rules for how to handle
> such things. That causes major pain for contributors ...

Yeah, I wish there was something more general.

>
> --
> Cheers,
>
> David / dhildenb
>

Cheers, Lorenzo

On 21.05.25 14:33, Lorenzo Stoakes wrote:
> Fundamentally I trust you to make sure this all goes correctly so let's not
> belabour the point or delay things here :)
> 
> So in that vein, Nico - I would sugesst for future respins adding a really
> clear bit to the header as David suggested :) also update the cover letter
> tests so it isn't reliant on a possibly ephemeral web link.
> 
> But otherwise let's proceed as was.

Right, and maybe only post this series if there was a major change, 
otherwise wait until the other thing is on it's way upstream.

> 
> On Wed, May 21, 2025 at 02:24:45PM +0200, David Hildenbrand wrote:
>>>>
>>>> Anyhow, to me the dependency is obvious, but I've followed the MM meeting
>>>> discussions, development etc.
>>>
>>> Right but is it clear to Andrew? I mean the cover letter was super unclear
>>> to me.
>>
>> I mean, assuming that it would not be clear to Andrew (and I think it is
>> clear to Andrew), I we would get CCed on these emails and could immediately
>> scream STOOOOOP :)
>>
>> And until this would hit mm-stable, a bit more time would pass.
>>
>>>
>>> What's to prevent things getting merged out of order?
>>
>> Fortunately, there are still people working here and not machines (at least,
>> that's what I hope).
> 
> Obligatory link to this :P
> 

It's scary how relevant that has become lately :D

> https://www.youtube.com/watch?v=5lsExRvJTAI

... fortunately, whenever I tell the chatbots that they are wrong (IOW, 
everytime I use them) they reply with "Oh yes, you are right." ... so 
far ...

-- 
Cheers,

David / dhildenb

On Thu, May 15, 2025 at 11:41 AM Nico Pache <npache@redhat.com> wrote:
>
> This series is a follow-up to [1], which adds mTHP support to khugepaged.
> mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> configs to make sense. Without it global="defer" and  mTHP="inherit" case
> is "undefined" behavior.
>
> We've seen cases were customers switching from RHEL7 to RHEL8 see a
> significant increase in the memory footprint for the same workloads.
>
> Through our investigations we found that a large contributing factor to
> the increase in RSS was an increase in THP usage.
>
> For workloads like MySQL, or when using allocators like jemalloc, it is
> often recommended to set /transparent_hugepages/enabled=never. This is
> in part due to performance degradations and increased memory waste.
>
> This series introduces enabled=defer, this setting acts as a middle
> ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> page fault handler will act normally, making a hugepage if possible. If
> the allocation is not MADV_HUGEPAGE, then the page fault handler will
> default to the base size allocation. The caveat is that khugepaged can
> still operate on pages that are not MADV_HUGEPAGE.
>
> This allows for three things... one, applications specifically designed to
> use hugepages will get them, and two, applications that don't use
> hugepages can still benefit from them without aggressively inserting
> THPs at every possible chance. This curbs the memory waste, and defers
> the use of hugepages to khugepaged. Khugepaged can then scan the memory
> for eligible collapsing. Lastly there is the added benefit for those who
> want THPs but experience higher latency PFs. Now you can get base page
> performance at the PF handler and Hugepage performance for those mappings
> after they collapse.
>
> Admins may want to lower max_ptes_none, if not, khugepaged may
> aggressively collapse single allocations into hugepages.
>
> TESTING:
> - Built for x86_64, aarch64, ppc64le, and s390x
> - selftests mm
> - In [1] I provided a script [2] that has multiple access patterns
> - lots of general use.
> - redis testing. This test was my original case for the defer mode. What I
>    was able to prove was that THP=always leads to increased max_latency
>    cases; hence why it is recommended to disable THPs for redis servers.
>    However with 'defer' we dont have the max_latency spikes and can still
>    get the system to utilize THPs. I further tested this with the mTHP
>    defer setting and found that redis (and probably other jmalloc users)
>    can utilize THPs via defer (+mTHP defer) without a large latency
>    penalty and some potential gains. I uploaded some mmtest results
>    here[3] which compares:
>        stock+thp=never
>        stock+(m)thp=always
>        khugepaged-mthp + defer (max_ptes_none=64)
>
>   The results show that (m)THPs can cause some throughput regression in
>   some cases, but also has gains in other cases. The mTHP+defer results
>   have more gains and less losses over the (m)THP=always case.
>
> V6 Changes:
> - nits
> - rebased dependent series and added review tags
>
> V5 Changes:
> - rebased dependent series
> - added reviewed-by tag on 2/4
>
> V4 Changes:
> - Minor Documentation fixes
> - rebased the dependent series [1] onto mm-unstable
>     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
>
> V3 Changes:
> - Combined the documentation commits into one, and moved a section to the
>   khugepaged mthp patchset
>
> V2 Changes:
> - base changes on mTHP khugepaged support
> - Fix selftests parsing issue
> - add mTHP defer option
> - add mTHP defer Documentation
>
> [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> [2] - https://gitlab.com/npache/khugepaged_mthp_test
> [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
>
> Nico Pache (4):
>   mm: defer THP insertion to khugepaged
>   mm: document (m)THP defer usage
>   khugepaged: add defer option to mTHP options
>   selftests: mm: add defer to thp setting parser
>
>  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
>  include/linux/huge_mm.h                    | 18 +++++-
>  mm/huge_memory.c                           | 69 +++++++++++++++++++---
>  mm/khugepaged.c                            |  8 +--
>  tools/testing/selftests/mm/thp_settings.c  |  1 +
>  tools/testing/selftests/mm/thp_settings.h  |  1 +
>  6 files changed, 106 insertions(+), 22 deletions(-)
>
> --
> 2.49.0
>
>

Hello Nico,

Upon reviewing the series, it occurred to me that BPF could solve this
more cleanly. Adding a 'tva_flags' parameter to the BPF hook would
handle this case and future scenarios without requiring new modes. The
BPF mode could then serve as a unified solution.

-- 
Regards
Yafang

On Tue, May 20, 2025 at 3:25 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> On Thu, May 15, 2025 at 11:41 AM Nico Pache <npache@redhat.com> wrote:
> >
> > This series is a follow-up to [1], which adds mTHP support to khugepaged.
> > mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> > configs to make sense. Without it global="defer" and  mTHP="inherit" case
> > is "undefined" behavior.
> >
> > We've seen cases were customers switching from RHEL7 to RHEL8 see a
> > significant increase in the memory footprint for the same workloads.
> >
> > Through our investigations we found that a large contributing factor to
> > the increase in RSS was an increase in THP usage.
> >
> > For workloads like MySQL, or when using allocators like jemalloc, it is
> > often recommended to set /transparent_hugepages/enabled=never. This is
> > in part due to performance degradations and increased memory waste.
> >
> > This series introduces enabled=defer, this setting acts as a middle
> > ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> > page fault handler will act normally, making a hugepage if possible. If
> > the allocation is not MADV_HUGEPAGE, then the page fault handler will
> > default to the base size allocation. The caveat is that khugepaged can
> > still operate on pages that are not MADV_HUGEPAGE.
> >
> > This allows for three things... one, applications specifically designed to
> > use hugepages will get them, and two, applications that don't use
> > hugepages can still benefit from them without aggressively inserting
> > THPs at every possible chance. This curbs the memory waste, and defers
> > the use of hugepages to khugepaged. Khugepaged can then scan the memory
> > for eligible collapsing. Lastly there is the added benefit for those who
> > want THPs but experience higher latency PFs. Now you can get base page
> > performance at the PF handler and Hugepage performance for those mappings
> > after they collapse.
> >
> > Admins may want to lower max_ptes_none, if not, khugepaged may
> > aggressively collapse single allocations into hugepages.
> >
> > TESTING:
> > - Built for x86_64, aarch64, ppc64le, and s390x
> > - selftests mm
> > - In [1] I provided a script [2] that has multiple access patterns
> > - lots of general use.
> > - redis testing. This test was my original case for the defer mode. What I
> >    was able to prove was that THP=always leads to increased max_latency
> >    cases; hence why it is recommended to disable THPs for redis servers.
> >    However with 'defer' we dont have the max_latency spikes and can still
> >    get the system to utilize THPs. I further tested this with the mTHP
> >    defer setting and found that redis (and probably other jmalloc users)
> >    can utilize THPs via defer (+mTHP defer) without a large latency
> >    penalty and some potential gains. I uploaded some mmtest results
> >    here[3] which compares:
> >        stock+thp=never
> >        stock+(m)thp=always
> >        khugepaged-mthp + defer (max_ptes_none=64)
> >
> >   The results show that (m)THPs can cause some throughput regression in
> >   some cases, but also has gains in other cases. The mTHP+defer results
> >   have more gains and less losses over the (m)THP=always case.
> >
> > V6 Changes:
> > - nits
> > - rebased dependent series and added review tags
> >
> > V5 Changes:
> > - rebased dependent series
> > - added reviewed-by tag on 2/4
> >
> > V4 Changes:
> > - Minor Documentation fixes
> > - rebased the dependent series [1] onto mm-unstable
> >     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
> >
> > V3 Changes:
> > - Combined the documentation commits into one, and moved a section to the
> >   khugepaged mthp patchset
> >
> > V2 Changes:
> > - base changes on mTHP khugepaged support
> > - Fix selftests parsing issue
> > - add mTHP defer option
> > - add mTHP defer Documentation
> >
> > [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> > [2] - https://gitlab.com/npache/khugepaged_mthp_test
> > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> >
> > Nico Pache (4):
> >   mm: defer THP insertion to khugepaged
> >   mm: document (m)THP defer usage
> >   khugepaged: add defer option to mTHP options
> >   selftests: mm: add defer to thp setting parser
> >
> >  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
> >  include/linux/huge_mm.h                    | 18 +++++-
> >  mm/huge_memory.c                           | 69 +++++++++++++++++++---
> >  mm/khugepaged.c                            |  8 +--
> >  tools/testing/selftests/mm/thp_settings.c  |  1 +
> >  tools/testing/selftests/mm/thp_settings.h  |  1 +
> >  6 files changed, 106 insertions(+), 22 deletions(-)
> >
> > --
> > 2.49.0
> >
> >
>
> Hello Nico,
>
> Upon reviewing the series, it occurred to me that BPF could solve this
> more cleanly. Adding a 'tva_flags' parameter to the BPF hook would
> handle this case and future scenarios without requiring new modes. The
> BPF mode could then serve as a unified solution.
Hi Yafang,

I dont see how this is the case? This would require users to
modify/add functionality rather than configuring the system in this
manner. What if BPF is not configured or being used? Having to use an
additional technology that requires precise configuration doesn't seem
cleaner.

Either way, thank you for taking a look into the series !

-- Nico
>
> --
> Regards
> Yafang
>

On Wed, May 21, 2025 at 6:19 PM Nico Pache <npache@redhat.com> wrote:
>
> On Tue, May 20, 2025 at 3:25 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > On Thu, May 15, 2025 at 11:41 AM Nico Pache <npache@redhat.com> wrote:
> > >
> > > This series is a follow-up to [1], which adds mTHP support to khugepaged.
> > > mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> > > configs to make sense. Without it global="defer" and  mTHP="inherit" case
> > > is "undefined" behavior.
> > >
> > > We've seen cases were customers switching from RHEL7 to RHEL8 see a
> > > significant increase in the memory footprint for the same workloads.
> > >
> > > Through our investigations we found that a large contributing factor to
> > > the increase in RSS was an increase in THP usage.
> > >
> > > For workloads like MySQL, or when using allocators like jemalloc, it is
> > > often recommended to set /transparent_hugepages/enabled=never. This is
> > > in part due to performance degradations and increased memory waste.
> > >
> > > This series introduces enabled=defer, this setting acts as a middle
> > > ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> > > page fault handler will act normally, making a hugepage if possible. If
> > > the allocation is not MADV_HUGEPAGE, then the page fault handler will
> > > default to the base size allocation. The caveat is that khugepaged can
> > > still operate on pages that are not MADV_HUGEPAGE.
> > >
> > > This allows for three things... one, applications specifically designed to
> > > use hugepages will get them, and two, applications that don't use
> > > hugepages can still benefit from them without aggressively inserting
> > > THPs at every possible chance. This curbs the memory waste, and defers
> > > the use of hugepages to khugepaged. Khugepaged can then scan the memory
> > > for eligible collapsing. Lastly there is the added benefit for those who
> > > want THPs but experience higher latency PFs. Now you can get base page
> > > performance at the PF handler and Hugepage performance for those mappings
> > > after they collapse.
> > >
> > > Admins may want to lower max_ptes_none, if not, khugepaged may
> > > aggressively collapse single allocations into hugepages.
> > >
> > > TESTING:
> > > - Built for x86_64, aarch64, ppc64le, and s390x
> > > - selftests mm
> > > - In [1] I provided a script [2] that has multiple access patterns
> > > - lots of general use.
> > > - redis testing. This test was my original case for the defer mode. What I
> > >    was able to prove was that THP=always leads to increased max_latency
> > >    cases; hence why it is recommended to disable THPs for redis servers.
> > >    However with 'defer' we dont have the max_latency spikes and can still
> > >    get the system to utilize THPs. I further tested this with the mTHP
> > >    defer setting and found that redis (and probably other jmalloc users)
> > >    can utilize THPs via defer (+mTHP defer) without a large latency
> > >    penalty and some potential gains. I uploaded some mmtest results
> > >    here[3] which compares:
> > >        stock+thp=never
> > >        stock+(m)thp=always
> > >        khugepaged-mthp + defer (max_ptes_none=64)
> > >
> > >   The results show that (m)THPs can cause some throughput regression in
> > >   some cases, but also has gains in other cases. The mTHP+defer results
> > >   have more gains and less losses over the (m)THP=always case.
> > >
> > > V6 Changes:
> > > - nits
> > > - rebased dependent series and added review tags
> > >
> > > V5 Changes:
> > > - rebased dependent series
> > > - added reviewed-by tag on 2/4
> > >
> > > V4 Changes:
> > > - Minor Documentation fixes
> > > - rebased the dependent series [1] onto mm-unstable
> > >     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
> > >
> > > V3 Changes:
> > > - Combined the documentation commits into one, and moved a section to the
> > >   khugepaged mthp patchset
> > >
> > > V2 Changes:
> > > - base changes on mTHP khugepaged support
> > > - Fix selftests parsing issue
> > > - add mTHP defer option
> > > - add mTHP defer Documentation
> > >
> > > [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> > > [2] - https://gitlab.com/npache/khugepaged_mthp_test
> > > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> > >
> > > Nico Pache (4):
> > >   mm: defer THP insertion to khugepaged
> > >   mm: document (m)THP defer usage
> > >   khugepaged: add defer option to mTHP options
> > >   selftests: mm: add defer to thp setting parser
> > >
> > >  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
> > >  include/linux/huge_mm.h                    | 18 +++++-
> > >  mm/huge_memory.c                           | 69 +++++++++++++++++++---
> > >  mm/khugepaged.c                            |  8 +--
> > >  tools/testing/selftests/mm/thp_settings.c  |  1 +
> > >  tools/testing/selftests/mm/thp_settings.h  |  1 +
> > >  6 files changed, 106 insertions(+), 22 deletions(-)
> > >
> > > --
> > > 2.49.0
> > >
> > >
> >
> > Hello Nico,
> >
> > Upon reviewing the series, it occurred to me that BPF could solve this
> > more cleanly. Adding a 'tva_flags' parameter to the BPF hook would
> > handle this case and future scenarios without requiring new modes. The
> > BPF mode could then serve as a unified solution.
> Hi Yafang,
>
> I dont see how this is the case? This would require users to
> modify/add functionality rather than configuring the system in this
> manner. What if BPF is not configured or being used? Having to use an
> additional technology that requires precise configuration doesn't seem
> cleaner.

The core challenge remains: while certain tasks benefit from this new
mode, others see no improvement—or may even regress.
For that reason, implementing it globally seems unwise—per-task
control would be far more effective.

-- 
Regards
Yafang