Re: [PATCH v2 06/15] migration: Yield bitmap_mutex properly when sending/sleeping

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: qemu-devel@nongnu.org,
	"Daniel P . Berrange" <berrange@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	ani@anisinha.ca,
	Leonardo Bras Soares Passos <lsoaresp@redhat.com>,
	Manish Mishra <manish.mishra@nutanix.com>
Subject: Re: [PATCH v2 06/15] migration: Yield bitmap_mutex properly when sending/sleeping
Date: Wed, 12 Oct 2022 17:43:53 +0100	[thread overview]
Message-ID: <Y0buySbboE3xOVoQ@work-vm> (raw)
In-Reply-To: <20221011215559.602584-7-peterx@redhat.com>

* 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