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

Dave Chinner
2021-08-05T06:43:29Z

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);
}

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);
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