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

From: Brian Foster <bfoster@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-xfs@vger.kernel.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 17/26] xfs: don't block kswapd in inode reclaim
Date: Fri, 11 Oct 2019 11:29:57 -0400
Message-ID: <20191011152957.GI61257@bfoster> (raw)
In-Reply-To: <20191009032124.10541-18-david@fromorbit.com>

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
>