Re: [PATCH 05/14] xfs: per-cpu deferred inode inactivation queues

Darrick J. Wong
2021-08-05T07:00:35Z

On Thu, Aug 05, 2021 at 04:43:24PM +1000, Dave Chinner wrote:
> On Wed, Aug 04, 2021 at 07:06:50PM -0700, Darrick J. Wong wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > Move inode inactivation to background work contexts so that it no
> > longer runs in the context that releases the final reference to an
> > inode. This will allow process work that ends up blocking on
> > inactivation to continue doing work while the filesytem processes
> > the inactivation in the background.
> >
> > A typical demonstration of this is unlinking an inode with lots of
> > extents. The extents are removed during inactivation, so this blocks
> > the process that unlinked the inode from the directory structure. By
> > moving the inactivation to the background process, the userspace
> > applicaiton can keep working (e.g. unlinking the next inode in the
> > directory) while the inactivation work on the previous inode is
> > done by a different CPU.
> >
> > The implementation of the queue is relatively simple. We use a
> > per-cpu lockless linked list (llist) to queue inodes for
> > inactivation without requiring serialisation mechanisms, and a work
> > item to allow the queue to be processed by a CPU bound worker
> > thread. We also keep a count of the queue depth so that we can
> > trigger work after a number of deferred inactivations have been
> > queued.
> >
> > The use of a bound workqueue with a single work depth allows the
> > workqueue to run one work item per CPU. We queue the work item on
> > the CPU we are currently running on, and so this essentially gives
> > us affine per-cpu worker threads for the per-cpu queues. THis
> > maintains the effective CPU affinity that occurs within XFS at the
> > AG level due to all objects in a directory being local to an AG.
> > Hence inactivation work tends to run on the same CPU that last
> > accessed all the objects that inactivation accesses and this
> > maintains hot CPU caches for unlink workloads.
> >
> > A depth of 32 inodes was chosen to match the number of inodes in an
> > inode cluster buffer. This hopefully allows sequential
> > allocation/unlink behaviours to defering inactivation of all the
> > inodes in a single cluster buffer at a time, further helping
> > maintain hot CPU and buffer cache accesses while running
> > inactivations.
> >
> > A hard per-cpu queue throttle of 256 inode has been set to avoid
> > runaway queuing when inodes that take a long to time inactivate are
> > being processed. For example, when unlinking inodes with large
> > numbers of extents that can take a lot of processing to free.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > [djwong: tweak comments and tracepoints, convert opflags to state bits]
> > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> .....
> > @@ -740,6 +751,25 @@ xfs_fs_sync_fs(
> > flush_delayed_work(&mp->m_log->l_work);
> > }
> >
> > + /*
> > + * Flush all deferred inode inactivation work so that the free space
> > + * counters will reflect recent deletions. Do not force the log again
> > + * because log recovery can restart the inactivation from the info that
> > + * we just wrote into the ondisk log.
> > + *
> > + * For regular operation this isn't strictly necessary since we aren't
> > + * required to guarantee that unlinking frees space immediately, but
> > + * that is how XFS historically behaved.
> > + *
> > + * If, however, the filesystem is at FREEZE_PAGEFAULTS, this is our
> > + * last chance to complete the inactivation work before the filesystem
> > + * freezes and the log is quiesced. The background worker will not
> > + * activate again until the fs is thawed because the VFS won't evict
> > + * any more inodes until freeze_super drops s_umount and we disable the
> > + * worker in xfs_fs_freeze.
> > + */
> > + xfs_inodegc_flush(mp);
> > +
> > return 0;
> > }
> >
> > @@ -854,6 +884,17 @@ xfs_fs_freeze(
> > */
> > flags = memalloc_nofs_save();
> > xfs_blockgc_stop(mp);
> > +
> > + /*
> > + * Stop the inodegc background worker. freeze_super already flushed
> > + * all pending inodegc work when it sync'd the filesystem after setting
> > + * SB_FREEZE_PAGEFAULTS, and it holds s_umount, so we know that inodes
> > + * cannot enter xfs_fs_destroy_inode until the freeze is complete.
> > + * If the filesystem is read-write, inactivated inodes will queue but
> > + * the worker will not run until the filesystem thaws or unmounts.
> > + */
> > + xfs_inodegc_stop(mp);
> > +
> > xfs_save_resvblks(mp);
> > ret = xfs_log_quiesce(mp);
> > memalloc_nofs_restore(flags);
>
> I still think this freeze handling is problematic. While I can't easily trigger
> the problem I saw, I still don't really see what makes the flush in
> xfs_fs_sync_fs() prevent races with the final stage of freeze before
> inactivation is stopped......
>
> .... and ....
>
> as I write this the xfs/517 loop goes boom on my pmem test setup (but no DAX):
>
> SECTION -- xfs
> FSTYP -- xfs (debug)
> PLATFORM -- Linux/x86_64 test3 5.14.0-rc4-dgc #506 SMP PREEMPT Thu Aug 5 15:49:49 AEST 2021
> MKFS_OPTIONS -- -f -m rmapbt=1 /dev/pmem1
> MOUNT_OPTIONS -- -o dax=never -o context=system_u:object_r:root_t:s0 /dev/pmem1 /mnt/scratch
>
> generic/390 3s ... 3s
> xfs/517 43s ...
> Message from syslogd@test3 at Aug 5 15:56:24 ...
> kernel:[ 162.849634] XFS: Assertion failed: mp->m_super->s_writers.frozen < SB_FREEZE_FS, file: fs/xfs/xfs_icache.c, line: 1889
>
> I suspect that we could actually target this better and close the
> race by doing something like:
>
> xfs_fs_sync_fs()
> {
> ....
>
> /*
> * If we are called with page faults frozen out, it means we are about
> * to freeze the transaction subsystem. Take the opportunity to shut
> * down inodegc because once SB_FREEZE_FS is set it's too late to
> * prevent inactivation races with freeze. The fs doesn't get called
> * again by the freezing process until after SB_FREEZE_FS has been set,
> * so it's now or never.
> *
> * We don't care if this is a normal syncfs call that does this or
> * freeze that does this - we can run this multiple times without issue
> * and we won't race with a restart because a restart can only occur when
> * the state is either SB_FREEZE_FS or SB_FREEZE_COMPLETE.
> */
> if (sb->s_writers.frozen == SB_FREEZE_PAGEFAULT)
> xfs_inodegc_stop(mp);

LOL, a previous version of this series actually did this part this way,
but...

> }
>
> xfs_fs_freeze()
> {
> .....
> error:
> /*
> * We need to restart the inodegc on error because we stopped it at
> * SB_FREEZE_PAGEFAULT level and a thaw is not going to be run to
> * restart it now. We are at SB_FREEZE_FS level here, so we can restart
> * safely without racing with a stop in xfs_fs_sync_fs().
> */
> if (error)
> xfs_inodegc_start(mp);

...missed this part. If this fixes x517 and doesn't break g390 for you,
I'll meld it into the series. I think the reasoning here makes sense.

--D

> return error:
> }
>
> The stop and "restart on error" are done under the same s_umount hold, so they
> are atomic w.r.t. to other freeze operations so doesn't have the problems with
> nested freeze/thaw (g/390) that my last patch had.
>
> Your thoughts?
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com