Don't take the bitmap mutex when sending pages, or when being throttled by
migration_rate_limit() (which is a bit tricky to call it here in ram code,
but seems still helpful).
It prepares for the possibility of concurrently sending pages in >1 threads
using the function ram_save_host_page() because all threads may need the
bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of
qemu_sem_wait() blocking for one thread will not block the other from
progressing.
Signed-off-by: Peter Xu <peterx@redhat.com>
---
migration/ram.c | 41 ++++++++++++++++++++++++++++++-----------
1 file changed, 30 insertions(+), 11 deletions(-)
diff --git a/migration/ram.c b/migration/ram.c
index b9ac2d6921..578ad8d70a 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2462,6 +2462,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs)
*/
static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
{
+ bool page_dirty, preempt_active = postcopy_preempt_active();
int tmppages, pages = 0;
size_t pagesize_bits =
qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
@@ -2485,22 +2486,40 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
break;
}
- /* Check the pages is dirty and if it is send it */
- if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
- tmppages = ram_save_target_page(rs, pss);
- if (tmppages < 0) {
- return tmppages;
- }
+ page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page);
- pages += tmppages;
+ /* Check the pages is dirty and if it is send it */
+ if (page_dirty) {
/*
- * Allow rate limiting to happen in the middle of huge pages if
- * something is sent in the current iteration.
+ * Properly yield the lock only in postcopy preempt mode
+ * because both migration thread and rp-return thread can
+ * operate on the bitmaps.
*/
- if (pagesize_bits > 1 && tmppages > 0) {
- migration_rate_limit();
+ if (preempt_active) {
+ qemu_mutex_unlock(&rs->bitmap_mutex);
+ }
+ tmppages = ram_save_target_page(rs, pss);
+ if (tmppages >= 0) {
+ pages += tmppages;
+ /*
+ * Allow rate limiting to happen in the middle of huge pages if
+ * something is sent in the current iteration.
+ */
+ if (pagesize_bits > 1 && tmppages > 0) {
+ migration_rate_limit();
+ }
}
+ if (preempt_active) {
+ qemu_mutex_lock(&rs->bitmap_mutex);
+ }
+ } else {
+ tmppages = 0;
}
+
+ if (tmppages < 0) {
+ return tmppages;
+ }
+
pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
} while ((pss->page < hostpage_boundary) &&
offset_in_ramblock(pss->block,
--
2.37.3* Peter Xu (peterx@redhat.com) wrote:
> Don't take the bitmap mutex when sending pages, or when being throttled by
> migration_rate_limit() (which is a bit tricky to call it here in ram code,
> but seems still helpful).
>
> It prepares for the possibility of concurrently sending pages in >1 threads
> using the function ram_save_host_page() because all threads may need the
> bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of
> qemu_sem_wait() blocking for one thread will not block the other from
> progressing.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
although a comment above the reclaration of ram_save_host_pages saying
it can drop the lock would be veyr good.
Dave
> ---
> migration/ram.c | 41 ++++++++++++++++++++++++++++++-----------
> 1 file changed, 30 insertions(+), 11 deletions(-)
>
> diff --git a/migration/ram.c b/migration/ram.c
> index b9ac2d6921..578ad8d70a 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2462,6 +2462,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs)
> */
> static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
> {
> + bool page_dirty, preempt_active = postcopy_preempt_active();
> int tmppages, pages = 0;
> size_t pagesize_bits =
> qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
> @@ -2485,22 +2486,40 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
> break;
> }
>
> - /* Check the pages is dirty and if it is send it */
> - if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
> - tmppages = ram_save_target_page(rs, pss);
> - if (tmppages < 0) {
> - return tmppages;
> - }
> + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page);
>
> - pages += tmppages;
> + /* Check the pages is dirty and if it is send it */
> + if (page_dirty) {
> /*
> - * Allow rate limiting to happen in the middle of huge pages if
> - * something is sent in the current iteration.
> + * Properly yield the lock only in postcopy preempt mode
> + * because both migration thread and rp-return thread can
> + * operate on the bitmaps.
> */
> - if (pagesize_bits > 1 && tmppages > 0) {
> - migration_rate_limit();
> + if (preempt_active) {
> + qemu_mutex_unlock(&rs->bitmap_mutex);
> + }
> + tmppages = ram_save_target_page(rs, pss);
> + if (tmppages >= 0) {
> + pages += tmppages;
> + /*
> + * Allow rate limiting to happen in the middle of huge pages if
> + * something is sent in the current iteration.
> + */
> + if (pagesize_bits > 1 && tmppages > 0) {
> + migration_rate_limit();
> + }
> }
> + if (preempt_active) {
> + qemu_mutex_lock(&rs->bitmap_mutex);
> + }
> + } else {
> + tmppages = 0;
> }
> +
> + if (tmppages < 0) {
> + return tmppages;
> + }
> +
> pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
> } while ((pss->page < hostpage_boundary) &&
> offset_in_ramblock(pss->block,
> --
> 2.37.3
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
On Wed, Oct 12, 2022 at 05:43:53PM +0100, Dr. David Alan Gilbert wrote: > * Peter Xu (peterx@redhat.com) wrote: > > Don't take the bitmap mutex when sending pages, or when being throttled by > > migration_rate_limit() (which is a bit tricky to call it here in ram code, > > but seems still helpful). > > > > It prepares for the possibility of concurrently sending pages in >1 threads > > using the function ram_save_host_page() because all threads may need the > > bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of > > qemu_sem_wait() blocking for one thread will not block the other from > > progressing. > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > although a comment above the reclaration of ram_save_host_pages saying > it can drop the lock would be veyr good. Let me add that. Thanks, -- Peter Xu
On Wed, Oct 12, 2022 at 01:51:07PM -0400, Peter Xu wrote: > On Wed, Oct 12, 2022 at 05:43:53PM +0100, Dr. David Alan Gilbert wrote: > > * Peter Xu (peterx@redhat.com) wrote: > > > Don't take the bitmap mutex when sending pages, or when being throttled by > > > migration_rate_limit() (which is a bit tricky to call it here in ram code, > > > but seems still helpful). > > > > > > It prepares for the possibility of concurrently sending pages in >1 threads > > > using the function ram_save_host_page() because all threads may need the > > > bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of > > > qemu_sem_wait() blocking for one thread will not block the other from > > > progressing. > > > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > > > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > > > although a comment above the reclaration of ram_save_host_pages saying > > it can drop the lock would be veyr good. > > Let me add that. Thanks, A fixup to this patch attached to touch up the comment for ram_save_host_page(). -- Peter Xu From dcc3adce062df7216851890d49f7d2b1fa2e84a4 Mon Sep 17 00:00:00 2001 From: Peter Xu <peterx@redhat.com> Date: Thu, 13 Oct 2022 12:18:04 -0400 Subject: [PATCH] fixup! migration: Yield bitmap_mutex properly when sending/sleeping Content-type: text/plain Signed-off-by: Peter Xu <peterx@redhat.com> --- migration/ram.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/migration/ram.c b/migration/ram.c index 538667b974..b311ece48c 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -2407,9 +2407,14 @@ out: * a host page in which case the remainder of the hostpage is sent. * Only dirty target pages are sent. Note that the host page size may * be a huge page for this block. + * * The saving stops at the boundary of the used_length of the block * if the RAMBlock isn't a multiple of the host page size. * + * The caller must be with ram_state.bitmap_mutex held to call this + * function. Note that this function can temporarily release the lock, but + * when the function is returned it'll make sure the lock is still held. + * * Returns the number of pages written or negative on error * * @rs: current RAM state -- 2.37.3
Peter Xu <peterx@redhat.com> wrote: > On Wed, Oct 12, 2022 at 01:51:07PM -0400, Peter Xu wrote: >> On Wed, Oct 12, 2022 at 05:43:53PM +0100, Dr. David Alan Gilbert wrote: >> > * Peter Xu (peterx@redhat.com) wrote: >> > > Don't take the bitmap mutex when sending pages, or when being throttled by >> > > migration_rate_limit() (which is a bit tricky to call it here in ram code, >> > > but seems still helpful). >> > > >> > > It prepares for the possibility of concurrently sending pages in >1 threads >> > > using the function ram_save_host_page() because all threads may need the >> > > bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of >> > > qemu_sem_wait() blocking for one thread will not block the other from >> > > progressing. >> > > >> > > Signed-off-by: Peter Xu <peterx@redhat.com> >> > >> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> >> > >> > although a comment above the reclaration of ram_save_host_pages saying >> > it can drop the lock would be veyr good. >> >> Let me add that. Thanks, > > A fixup to this patch attached to touch up the comment for > ram_save_host_page(). Reviewed-by: Juan Quintela <quintela@redhat.com>
* Peter Xu (peterx@redhat.com) wrote: > On Wed, Oct 12, 2022 at 01:51:07PM -0400, Peter Xu wrote: > > On Wed, Oct 12, 2022 at 05:43:53PM +0100, Dr. David Alan Gilbert wrote: > > > * Peter Xu (peterx@redhat.com) wrote: > > > > Don't take the bitmap mutex when sending pages, or when being throttled by > > > > migration_rate_limit() (which is a bit tricky to call it here in ram code, > > > > but seems still helpful). > > > > > > > > It prepares for the possibility of concurrently sending pages in >1 threads > > > > using the function ram_save_host_page() because all threads may need the > > > > bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of > > > > qemu_sem_wait() blocking for one thread will not block the other from > > > > progressing. > > > > > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > > > > > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > > > > > although a comment above the reclaration of ram_save_host_pages saying > > > it can drop the lock would be veyr good. > > > > Let me add that. Thanks, > > A fixup to this patch attached to touch up the comment for > ram_save_host_page(). Yep, that's right (I don't think we have any formal annotation for locks) Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > -- > Peter Xu > From dcc3adce062df7216851890d49f7d2b1fa2e84a4 Mon Sep 17 00:00:00 2001 > From: Peter Xu <peterx@redhat.com> > Date: Thu, 13 Oct 2022 12:18:04 -0400 > Subject: [PATCH] fixup! migration: Yield bitmap_mutex properly when > sending/sleeping > Content-type: text/plain > > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > migration/ram.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/migration/ram.c b/migration/ram.c > index 538667b974..b311ece48c 100644 > --- a/migration/ram.c > +++ b/migration/ram.c > @@ -2407,9 +2407,14 @@ out: > * a host page in which case the remainder of the hostpage is sent. > * Only dirty target pages are sent. Note that the host page size may > * be a huge page for this block. > + * > * The saving stops at the boundary of the used_length of the block > * if the RAMBlock isn't a multiple of the host page size. > * > + * The caller must be with ram_state.bitmap_mutex held to call this > + * function. Note that this function can temporarily release the lock, but > + * when the function is returned it'll make sure the lock is still held. > + * > * Returns the number of pages written or negative on error > * > * @rs: current RAM state > -- > 2.37.3 > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
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