Re: [PATCH 17/24] xfs: don't block kswapd in inode reclaim

Brian Foster
2019-08-07T11:14:12Z

On Wed, Aug 07, 2019 at 07:27:04AM +1000, Dave Chinner wrote:
> On Tue, Aug 06, 2019 at 02:21:31PM -0400, Brian Foster wrote:
> > On Thu, Aug 01, 2019 at 12:17:45PM +1000, 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.
> > >
> > > Signed-off-by: Dave Chinner <dchinner@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 0b0fd10a36d4..2fa2f8dcf86b 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);
> >
> > So we're unblocking kswapd from dirty items, but we already kick the AIL
> > regardless of kswapd or not in inode reclaim. Why the change to no
> > longer kick the AIL in the !kswapd case? Whatever the reasoning, a
> > mention in the commit log would be helpful...
>
> Because we used to block reclaim, we never knew how long it would be
> before it came back (say it had to write 1024 inode buffers), so
> every time we entered reclaim here we kicked the AIL in case we did
> get blocked for a long time.
>
> Now kswapd doesn't block at all, we know it's going to enter this
> code repeatedly while direct reclaim is blocked, and so we only need
> it to kick background inode writeback via kswapd rather than all
> reclaim now.
>

Got it. Can you include this reasoning in the commit log description
please?

Brian

> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com