To start I figured I'm going to bench about as friendly case as it gets
-- statically linked *separate* binaries all doing execve in a loop.

I borrowed the bench from here:
http://apollo.backplane.com/DFlyMisc/doexec.c

$ cc -static -O2 -o static-doexec doexec.c
$ ./static-doexec $(nproc)

It prints a result every second.

My test box is temporarily only 26 cores and even at this scale I run
into massive lock contention stemming from back-to-back calls to
percpu_counter_init (and _destroy later).

While not a panacea, one simple thing to do here is to batch these ops.
Since the term "batching" is already used in the file, I decided to
refer to it as "grouping" instead.

Even if this code could be patched to dodge these counters,  I would
argue a high-traffic alloc/free consumer is only a matter of time so it
makes sense to facilitate it.

With the fix I get an ok win, to quote from the commit:
> Even at a very modest scale of 26 cores (ops/s):
> before: 133543.63
> after:  186061.81 (+39%)

While with the patch these allocations remain a significant problem,
the primary bottleneck shifts to:

    __pv_queued_spin_lock_slowpath+1
    _raw_spin_lock_irqsave+57
    folio_lruvec_lock_irqsave+91
    release_pages+590
    tlb_batch_pages_flush+61
    tlb_finish_mmu+101
    exit_mmap+327
    __mmput+61
    begin_new_exec+1245
    load_elf_binary+712
    bprm_execve+644
    do_execveat_common.isra.0+429
    __x64_sys_execve+50
    do_syscall_64+46
    entry_SYSCALL_64_after_hwframe+110

I intend to do more work on the area to mostly sort it out, but I would
not mind if someone else took the hammer to folio. :)

With this out of the way I'll be looking at some form of caching to
eliminate these allocs as a problem.

v3:
- fix !CONFIG_SMP build
- drop the backtrace from fork commit message

v2:
- force bigger alignment on alloc
- rename "counters" to "nr_counters" and pass prior to lock key
- drop {}'s for single-statement loops


Mateusz Guzik (2):
  pcpcntr: add group allocation/free
  fork: group allocation of per-cpu counters for mm struct

 include/linux/percpu_counter.h | 39 ++++++++++++++++++----
 kernel/fork.c                  | 14 ++------
 lib/percpu_counter.c           | 61 +++++++++++++++++++++++-----------
 3 files changed, 77 insertions(+), 37 deletions(-)

-- 
2.41.0

Hello,

On Wed, Aug 23, 2023 at 07:06:07AM +0200, Mateusz Guzik wrote:
> To start I figured I'm going to bench about as friendly case as it gets
> -- statically linked *separate* binaries all doing execve in a loop.
> 
> I borrowed the bench from here:
> http://apollo.backplane.com/DFlyMisc/doexec.c
> 
> $ cc -static -O2 -o static-doexec doexec.c
> $ ./static-doexec $(nproc)
> 
> It prints a result every second.
> 
> My test box is temporarily only 26 cores and even at this scale I run
> into massive lock contention stemming from back-to-back calls to
> percpu_counter_init (and _destroy later).
> 
> While not a panacea, one simple thing to do here is to batch these ops.
> Since the term "batching" is already used in the file, I decided to
> refer to it as "grouping" instead.
> 
> Even if this code could be patched to dodge these counters,  I would
> argue a high-traffic alloc/free consumer is only a matter of time so it
> makes sense to facilitate it.
> 
> With the fix I get an ok win, to quote from the commit:
> > Even at a very modest scale of 26 cores (ops/s):
> > before: 133543.63
> > after:  186061.81 (+39%)
> 
> While with the patch these allocations remain a significant problem,
> the primary bottleneck shifts to:
> 
>     __pv_queued_spin_lock_slowpath+1
>     _raw_spin_lock_irqsave+57
>     folio_lruvec_lock_irqsave+91
>     release_pages+590
>     tlb_batch_pages_flush+61
>     tlb_finish_mmu+101
>     exit_mmap+327
>     __mmput+61
>     begin_new_exec+1245
>     load_elf_binary+712
>     bprm_execve+644
>     do_execveat_common.isra.0+429
>     __x64_sys_execve+50
>     do_syscall_64+46
>     entry_SYSCALL_64_after_hwframe+110
> 
> I intend to do more work on the area to mostly sort it out, but I would
> not mind if someone else took the hammer to folio. :)
> 
> With this out of the way I'll be looking at some form of caching to
> eliminate these allocs as a problem.
> 
> v3:
> - fix !CONFIG_SMP build
> - drop the backtrace from fork commit message
> 
> v2:
> - force bigger alignment on alloc
> - rename "counters" to "nr_counters" and pass prior to lock key
> - drop {}'s for single-statement loops
> 
> 
> Mateusz Guzik (2):
>   pcpcntr: add group allocation/free
>   fork: group allocation of per-cpu counters for mm struct
> 
>  include/linux/percpu_counter.h | 39 ++++++++++++++++++----
>  kernel/fork.c                  | 14 ++------
>  lib/percpu_counter.c           | 61 +++++++++++++++++++++++-----------
>  3 files changed, 77 insertions(+), 37 deletions(-)
> 
> -- 
> 2.41.0
> 

I've applied both to percpu#for-6.6.

Thanks,
Dennis