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

["Dr. David Alan Gilbert" <dgilbert@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>

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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK