* Peter Xu (pet...@redhat.com) wrote:
> On Tue, Oct 04, 2022 at 02:55:10PM +0100, Dr. David Alan Gilbert wrote:
> > * Peter Xu (pet...@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 <pet...@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
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
