Re: [PATCH 18/24] xfs: reduce kswapd blocking on inode locking.

Brian Foster
2019-08-06T18:22:15Z

On Thu, Aug 01, 2019 at 12:17:46PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> When doing async node reclaiming, we grab a batch of inodes that we
> are likely able to reclaim and ignore those that are already
> flushing. However, when we actually go to reclaim them, the first
> thing we do is lock the inode. If we are racing with something
> else reclaiming the inode or flushing it because it is dirty,
> we block on the inode lock. Hence we can still block kswapd here.
>
> Further, if we flush an inode, we also cluster all the other dirty
> inodes in that cluster into the same IO, flush locking them all.
> However, if the workload is operating on sequential inodes (e.g.
> created by a tarball extraction) most of these inodes will be
> sequntial in the cache and so in the same batch
> we've already grabbed for reclaim scanning.
>
> As a result, it is common for all the inodes in the batch to be
> dirty and it is common for the first inode flushed to also flush all
> the inodes in the reclaim batch. In which case, they are now all
> going to be flush locked and we do not want to block on them.
>

Hmm... I think I'm missing something with this description. For dirty
inodes that are flushed in a cluster via reclaim as described, aren't we
already blocking on all of the flush locks by virtue of the synchronous
I/O associated with the flush of the first dirty inode in that
particular cluster?

Brian

> Hence, for async reclaim (SYNC_TRYLOCK) make sure we always use
> trylock semantics and abort reclaim of an inode as quickly as we can
> without blocking kswapd.
>
> Found via tracing and finding big batches of repeated lock/unlock
> runs on inodes that we just flushed by write clustering during
> reclaim.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_icache.c | 23 ++++++++++++++++++-----
> 1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 2fa2f8dcf86b..e6b9030875b9 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -1104,11 +1104,23 @@ xfs_reclaim_inode(
>
> restart:
> error = 0;
> - xfs_ilock(ip, XFS_ILOCK_EXCL);
> - if (!xfs_iflock_nowait(ip)) {
> - if (!(sync_mode & SYNC_WAIT))
> + /*
> + * Don't try to flush the inode if another inode in this cluster has
> + * already flushed it after we did the initial checks in
> + * xfs_reclaim_inode_grab().
> + */
> + if (sync_mode & SYNC_TRYLOCK) {
> + if (!xfs_ilock_nowait(ip, XFS_ILOCK_EXCL))
> goto out;
> - xfs_iflock(ip);
> + if (!xfs_iflock_nowait(ip))
> + goto out_unlock;
> + } else {
> + xfs_ilock(ip, XFS_ILOCK_EXCL);
> + if (!xfs_iflock_nowait(ip)) {
> + if (!(sync_mode & SYNC_WAIT))
> + goto out_unlock;
> + xfs_iflock(ip);
> + }
> }
>
> if (XFS_FORCED_SHUTDOWN(ip->i_mount)) {
> @@ -1215,9 +1227,10 @@ xfs_reclaim_inode(
>
> out_ifunlock:
> xfs_ifunlock(ip);
> +out_unlock:
> + xfs_iunlock(ip, XFS_ILOCK_EXCL);
> out:
> xfs_iflags_clear(ip, XFS_IRECLAIM);
> - xfs_iunlock(ip, XFS_ILOCK_EXCL);
> /*
> * We could return -EAGAIN here to make reclaim rescan the inode tree in
> * a short while. However, this just burns CPU time scanning the tree
> --
> 2.22.0
>