Re: [RFC PATCH] migration/ram: avoid to do log clear in the last round

[Fabiano Rosas <farosas@suse.de>]

Yanfei Xu <yanfei.xu@bytedance.com> writes:

> There won't be any ram sync after the stage of save_complete, therefore
> it's unnecessary to do manually protect for dirty pages being sent. Skip
> to do this in last round can reduce noticeable downtime.
>
> Signed-off-by: Yanfei Xu <yanfei.xu@bytedance.com>
> ---
> As I don't have proper machine to test this patch in qemu and verify if it has
> risks like in postcopy, colo and so on.(But I tested this idea on my rust VMM,
> it works and can reduce ~50ms for a 128GB guest). So I raise the patch with RFC
> for suggestions.
>
>  migration/ram.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/migration/ram.c b/migration/ram.c
> index e12913b43e..2b169cdd18 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -838,7 +838,9 @@ static inline bool migration_bitmap_clear_dirty(RAMState *rs,
>       * the page in the chunk we clear the remote dirty bitmap for all.
>       * Clearing it earlier won't be a problem, but too late will.
>       */
> -    migration_clear_memory_region_dirty_bitmap(rb, page);
> +    if (!rs->last_stage) {
> +        migration_clear_memory_region_dirty_bitmap(rb, page);
> +    }
>  
>      ret = test_and_clear_bit(page, rb->bmap);
>      if (ret) {

Well, it looks ok to me and passes all my tests.

Tested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>