Re: [PATCH 17/26] xfs: don't block kswapd in inode reclaim

Brian Foster
2019-10-11T15:30:02Z

On Wed, Oct 09, 2019 at 02:21:15PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> We have a number of reasons for blocking kswapd in XFS inode
> reclaim, mainly all to do with the fact that memory reclaim has no
> feedback mechanisms to throttle on dirty slab objects that need IO
> to reclaim.
>
> As a result, we currently throttle inode reclaim by issuing IO in
> the reclaim context. The unfortunate side effect of this is that it
> can cause long tail latencies in reclaim and for some workloads this
> can be a problem.
>
> Now that the shrinkers finally have a method of telling kswapd to
> back off, we can start the process of making inode reclaim in XFS
> non-blocking. The first thing we need to do is not block kswapd, but
> so that doesn't cause immediate serious problems, make sure inode
> writeback is always underway when kswapd is running.
>
> As we don't block kswapd now, we don't have to worry about reclaim
> scans taking long delays due to IO being issued and waited for.
> Hence while direct reclaim gets delayed by IO, kswapd will not and
> so it will keep pushing the AIL to clean inodes. Hence direct
> reclaim doesn't need to push the AIL anymore as kswapd will do it
> reliably now.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

> fs/xfs/xfs_icache.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 944add5ff8e0..edcc3f6bb3bf 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -1378,11 +1378,22 @@ xfs_reclaim_inodes_nr(
> struct xfs_mount *mp,
> int nr_to_scan)
> {
> - /* kick background reclaimer and push the AIL */
> + int sync_mode = SYNC_TRYLOCK;
> +
> + /* kick background reclaimer */
> xfs_reclaim_work_queue(mp);
> - xfs_ail_push_all(mp->m_ail);
>
> - return xfs_reclaim_inodes_ag(mp, SYNC_TRYLOCK | SYNC_WAIT, &nr_to_scan);
> + /*
> + * For kswapd, we kick background inode writeback. For direct
> + * reclaim, we issue and wait on inode writeback to throttle
> + * reclaim rates and avoid shouty OOM-death.
> + */
> + if (current_is_kswapd())
> + xfs_ail_push_all(mp->m_ail);
> + else
> + sync_mode |= SYNC_WAIT;
> +
> + return xfs_reclaim_inodes_ag(mp, sync_mode, &nr_to_scan);
> }
>
> /*
> --
> 2.23.0.rc1
>