Re: [PATCH 05/14] migration: Yield bitmap_mutex properly when sending/sleeping

[Peter Xu]

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