| Message ID | 20220920225210.48732-1-peterx@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | migration: Postcopy Preempt-Full | expand |
* 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> I generally dont like taking locks conditionally; but this kind of looks OK; I think it needs a big comment on the start of the function saying that it's called and left with the lock held but that it might drop it temporarily. > --- > migration/ram.c | 42 +++++++++++++++++++++++++++++++----------- > 1 file changed, 31 insertions(+), 11 deletions(-) > > diff --git a/migration/ram.c b/migration/ram.c > index 8303252b6d..6e7de6087a 100644 > --- a/migration/ram.c > +++ b/migration/ram.c > @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) > */ > static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > { > + bool page_dirty, release_lock = postcopy_preempt_active(); Could you rename that to something like 'drop_lock' - you are taking the lock at the end even when you have 'release_lock' set - which is a bit strange naming. > int tmppages, pages = 0; > size_t pagesize_bits = > qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; > @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > break; > } > > + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); > + /* > + * Properly yield the lock only in postcopy preempt mode because > + * both migration thread and rp-return thread can operate on the > + * bitmaps. > + */ > + if (release_lock) { > + qemu_mutex_unlock(&rs->bitmap_mutex); > + } Shouldn't the unlock/lock move inside the 'if (page_dirty) {' ? > /* Check the pages is dirty and if it is send it */ > - if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) { > + if (page_dirty) { > tmppages = ram_save_target_page(rs, pss); > - if (tmppages < 0) { > - return tmppages; > + 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(); This feels interesting, I know it's no change from before, and it's difficult to do here, but it seems odd to hold the lock around the sleeping in the rate limit. Dave > + } > } > + } else { > + 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 (release_lock) { > + qemu_mutex_lock(&rs->bitmap_mutex); > } > + > + 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, > -- > 2.32.0 >
On Tue, Oct 04, 2022 at 02:55:10PM +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> > > I generally dont like taking locks conditionally; but this kind of looks > OK; I think it needs a big comment on the start of the function saying > that it's called and left with the lock held but that it might drop it > temporarily. Right, the code is slightly hard to read, I just didn't yet see a good and easy solution for it yet. It's just that we may still want to keep the lock as long as possible for precopy in one shot. > > > --- > > migration/ram.c | 42 +++++++++++++++++++++++++++++++----------- > > 1 file changed, 31 insertions(+), 11 deletions(-) > > > > diff --git a/migration/ram.c b/migration/ram.c > > index 8303252b6d..6e7de6087a 100644 > > --- a/migration/ram.c > > +++ b/migration/ram.c > > @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) > > */ > > static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > { > > + bool page_dirty, release_lock = postcopy_preempt_active(); > > Could you rename that to something like 'drop_lock' - you are taking the > lock at the end even when you have 'release_lock' set - which is a bit > strange naming. Is there any difference on "drop" or "release"? I'll change the name anyway since I definitely trust you on any English comments, but please still let me know - I love to learn more on those! :) > > > int tmppages, pages = 0; > > size_t pagesize_bits = > > qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; > > @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > break; > > } > > > > + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); > > + /* > > + * Properly yield the lock only in postcopy preempt mode because > > + * both migration thread and rp-return thread can operate on the > > + * bitmaps. > > + */ > > + if (release_lock) { > > + qemu_mutex_unlock(&rs->bitmap_mutex); > > + } > > Shouldn't the unlock/lock move inside the 'if (page_dirty) {' ? I think we can move into it, but it may not be as optimal as keeping it as-is. Consider a case where we've got the bitmap with continous zero bits. During postcopy, the migration thread could be spinning here with the lock held even if it doesn't send a thing. It could still block the other return path thread on sending urgent pages which may be outside the zero zones. > > > > /* Check the pages is dirty and if it is send it */ > > - if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) { > > + if (page_dirty) { > > tmppages = ram_save_target_page(rs, pss); > > - if (tmppages < 0) { > > - return tmppages; > > + 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(); > > This feels interesting, I know it's no change from before, and it's > difficult to do here, but it seems odd to hold the lock around the > sleeping in the rate limit. Good point.. I think I'll leave it there for this patch because it's totally irrelevant, but seems proper in the future to do unlocking too for normal precopy. Maybe I'll just attach a patch at the end of this series when I repost. That'll be easier before things got forgotten again.
* Peter Xu (peterx@redhat.com) wrote: > On Tue, Oct 04, 2022 at 02:55:10PM +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> > > > > I generally dont like taking locks conditionally; but this kind of looks > > OK; I think it needs a big comment on the start of the function saying > > that it's called and left with the lock held but that it might drop it > > temporarily. > > Right, the code is slightly hard to read, I just didn't yet see a good and > easy solution for it yet. It's just that we may still want to keep the > lock as long as possible for precopy in one shot. > > > > > > --- > > > migration/ram.c | 42 +++++++++++++++++++++++++++++++----------- > > > 1 file changed, 31 insertions(+), 11 deletions(-) > > > > > > diff --git a/migration/ram.c b/migration/ram.c > > > index 8303252b6d..6e7de6087a 100644 > > > --- a/migration/ram.c > > > +++ b/migration/ram.c > > > @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) > > > */ > > > static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > { > > > + bool page_dirty, release_lock = postcopy_preempt_active(); > > > > Could you rename that to something like 'drop_lock' - you are taking the > > lock at the end even when you have 'release_lock' set - which is a bit > > strange naming. > > Is there any difference on "drop" or "release"? I'll change the name > anyway since I definitely trust you on any English comments, but please > still let me know - I love to learn more on those! :) I'm not sure 'drop' is much better either; I was struggling to find a good nam. > > > > > int tmppages, pages = 0; > > > size_t pagesize_bits = > > > qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; > > > @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > break; > > > } > > > > > > + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); > > > + /* > > > + * Properly yield the lock only in postcopy preempt mode because > > > + * both migration thread and rp-return thread can operate on the > > > + * bitmaps. > > > + */ > > > + if (release_lock) { > > > + qemu_mutex_unlock(&rs->bitmap_mutex); > > > + } > > > > Shouldn't the unlock/lock move inside the 'if (page_dirty) {' ? > > I think we can move into it, but it may not be as optimal as keeping it > as-is. > > Consider a case where we've got the bitmap with continous zero bits. > During postcopy, the migration thread could be spinning here with the lock > held even if it doesn't send a thing. It could still block the other > return path thread on sending urgent pages which may be outside the zero > zones. OK, that reason needs commenting then - you're going to do a lot of release/take pairs in that case which is going to show up as very hot; so hmm, if ti was just for that type of 'yield' behaviour you wouldn't normally do it for each bit. > > > > > > > /* Check the pages is dirty and if it is send it */ > > > - if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) { > > > + if (page_dirty) { > > > tmppages = ram_save_target_page(rs, pss); > > > - if (tmppages < 0) { > > > - return tmppages; > > > + 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(); > > > > This feels interesting, I know it's no change from before, and it's > > difficult to do here, but it seems odd to hold the lock around the > > sleeping in the rate limit. > > Good point.. I think I'll leave it there for this patch because it's > totally irrelevant, but seems proper in the future to do unlocking too for > normal precopy. > > Maybe I'll just attach a patch at the end of this series when I repost. > That'll be easier before things got forgotten again. Dave > -- > Peter Xu >
On Wed, Oct 05, 2022 at 12:18:00PM +0100, Dr. David Alan Gilbert wrote: > * Peter Xu (peterx@redhat.com) wrote: > > On Tue, Oct 04, 2022 at 02:55:10PM +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> > > > > > > I generally dont like taking locks conditionally; but this kind of looks > > > OK; I think it needs a big comment on the start of the function saying > > > that it's called and left with the lock held but that it might drop it > > > temporarily. > > > > Right, the code is slightly hard to read, I just didn't yet see a good and > > easy solution for it yet. It's just that we may still want to keep the > > lock as long as possible for precopy in one shot. > > > > > > > > > --- > > > > migration/ram.c | 42 +++++++++++++++++++++++++++++++----------- > > > > 1 file changed, 31 insertions(+), 11 deletions(-) > > > > > > > > diff --git a/migration/ram.c b/migration/ram.c > > > > index 8303252b6d..6e7de6087a 100644 > > > > --- a/migration/ram.c > > > > +++ b/migration/ram.c > > > > @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) > > > > */ > > > > static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > > { > > > > + bool page_dirty, release_lock = postcopy_preempt_active(); > > > > > > Could you rename that to something like 'drop_lock' - you are taking the > > > lock at the end even when you have 'release_lock' set - which is a bit > > > strange naming. > > > > Is there any difference on "drop" or "release"? I'll change the name > > anyway since I definitely trust you on any English comments, but please > > still let me know - I love to learn more on those! :) > > I'm not sure 'drop' is much better either; I was struggling to find a > good nam. I can also call it "preempt_enabled". Actually I can directly replace it with calling postcopy_preempt_active() always but I just want to make it crystal clear that the value is not changing and lock & unlock are always paired - in our case I think it is not changing, but the var helps to be 100% sure there'll be no possible bug on e.g. deadlock caused by state changing. > > > > > > > > int tmppages, pages = 0; > > > > size_t pagesize_bits = > > > > qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; > > > > @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > > break; > > > > } > > > > > > > > + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); > > > > + /* > > > > + * Properly yield the lock only in postcopy preempt mode because > > > > + * both migration thread and rp-return thread can operate on the > > > > + * bitmaps. > > > > + */ > > > > + if (release_lock) { > > > > + qemu_mutex_unlock(&rs->bitmap_mutex); > > > > + } > > > > > > Shouldn't the unlock/lock move inside the 'if (page_dirty) {' ? > > > > I think we can move into it, but it may not be as optimal as keeping it > > as-is. > > > > Consider a case where we've got the bitmap with continous zero bits. > > During postcopy, the migration thread could be spinning here with the lock > > held even if it doesn't send a thing. It could still block the other > > return path thread on sending urgent pages which may be outside the zero > > zones. > > OK, that reason needs commenting then - you're going to do a lot of > release/take pairs in that case which is going to show up as very hot; > so hmm, if ti was just for that type of 'yield' behaviour you wouldn't > normally do it for each bit. Hold on.. I think my assumption won't easily trigger, because at the end of the loop we'll try to look for the next "dirty" page. So continuously clean pages are unlikely, or I even think it's impossible because we're holding the mutex during scanning and clear-dirty, so no one will be able to flip the bit. So yeah I think it's okay to move it into "page_dirty", but since we'll mostly always go into dirty maybe it's just that it won't help a lot either, because it'll be mostly the same as keeping it outside?
On Wed, Oct 05, 2022 at 09:40:53AM -0400, Peter Xu wrote: > On Wed, Oct 05, 2022 at 12:18:00PM +0100, Dr. David Alan Gilbert wrote: > > * Peter Xu (peterx@redhat.com) wrote: > > > On Tue, Oct 04, 2022 at 02:55:10PM +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> > > > > > > > > I generally dont like taking locks conditionally; but this kind of looks > > > > OK; I think it needs a big comment on the start of the function saying > > > > that it's called and left with the lock held but that it might drop it > > > > temporarily. > > > > > > Right, the code is slightly hard to read, I just didn't yet see a good and > > > easy solution for it yet. It's just that we may still want to keep the > > > lock as long as possible for precopy in one shot. > > > > > > > > > > > > --- > > > > > migration/ram.c | 42 +++++++++++++++++++++++++++++++----------- > > > > > 1 file changed, 31 insertions(+), 11 deletions(-) > > > > > > > > > > diff --git a/migration/ram.c b/migration/ram.c > > > > > index 8303252b6d..6e7de6087a 100644 > > > > > --- a/migration/ram.c > > > > > +++ b/migration/ram.c > > > > > @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) > > > > > */ > > > > > static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > > > { > > > > > + bool page_dirty, release_lock = postcopy_preempt_active(); > > > > > > > > Could you rename that to something like 'drop_lock' - you are taking the > > > > lock at the end even when you have 'release_lock' set - which is a bit > > > > strange naming. > > > > > > Is there any difference on "drop" or "release"? I'll change the name > > > anyway since I definitely trust you on any English comments, but please > > > still let me know - I love to learn more on those! :) > > > > I'm not sure 'drop' is much better either; I was struggling to find a > > good nam. > > I can also call it "preempt_enabled". > > Actually I can directly replace it with calling postcopy_preempt_active() > always but I just want to make it crystal clear that the value is not > changing and lock & unlock are always paired - in our case I think it is > not changing, but the var helps to be 100% sure there'll be no possible bug > on e.g. deadlock caused by state changing. > > > > > > > > > > > > int tmppages, pages = 0; > > > > > size_t pagesize_bits = > > > > > qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; > > > > > @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > > > break; > > > > > } > > > > > > > > > > + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); > > > > > + /* > > > > > + * Properly yield the lock only in postcopy preempt mode because > > > > > + * both migration thread and rp-return thread can operate on the > > > > > + * bitmaps. > > > > > + */ > > > > > + if (release_lock) { > > > > > + qemu_mutex_unlock(&rs->bitmap_mutex); > > > > > + } > > > > > > > > Shouldn't the unlock/lock move inside the 'if (page_dirty) {' ? > > > > > > I think we can move into it, but it may not be as optimal as keeping it > > > as-is. > > > > > > Consider a case where we've got the bitmap with continous zero bits. > > > During postcopy, the migration thread could be spinning here with the lock > > > held even if it doesn't send a thing. It could still block the other > > > return path thread on sending urgent pages which may be outside the zero > > > zones. > > > > OK, that reason needs commenting then - you're going to do a lot of > > release/take pairs in that case which is going to show up as very hot; > > so hmm, if ti was just for that type of 'yield' behaviour you wouldn't > > normally do it for each bit. > > Hold on.. I think my assumption won't easily trigger, because at the end of > the loop we'll try to look for the next "dirty" page. So continuously > clean pages are unlikely, or I even think it's impossible because we're > holding the mutex during scanning and clear-dirty, so no one will be able > to flip the bit. > > So yeah I think it's okay to move it into "page_dirty", but since we'll > mostly always go into dirty maybe it's just that it won't help a lot > either, because it'll be mostly the same as keeping it outside? IOW, maybe I should drop page_dirty directly and replace it with a check, failing migration if migration_bitmap_clear_dirty() returned false?
diff --git a/migration/ram.c b/migration/ram.c index 8303252b6d..6e7de6087a 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) */ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) { + bool page_dirty, release_lock = postcopy_preempt_active(); int tmppages, pages = 0; size_t pagesize_bits = qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) break; } + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); + /* + * Properly yield the lock only in postcopy preempt mode because + * both migration thread and rp-return thread can operate on the + * bitmaps. + */ + if (release_lock) { + qemu_mutex_unlock(&rs->bitmap_mutex); + } + /* Check the pages is dirty and if it is send it */ - if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) { + if (page_dirty) { tmppages = ram_save_target_page(rs, pss); - if (tmppages < 0) { - return tmppages; + 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(); + } } + } else { + 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 (release_lock) { + qemu_mutex_lock(&rs->bitmap_mutex); } + + 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,
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 | 42 +++++++++++++++++++++++++++++++----------- 1 file changed, 31 insertions(+), 11 deletions(-)