On Wed, May 20, 2020 at 08:23:10AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> It's a global atomic counter, and we are hitting it at a rate of
> half a million transactions a second, so it's bouncing the counter
> cacheline all over the place on large machines. We don't actually
> need it anymore - it used to be required because the VFS freeze code
> could not track/prevent filesystem transactions that were running,
> but that problem no longer exists.
>
> Hence to remove the counter, we simply have to ensure that nothing
> calls xfs_sync_sb() while we are trying to quiesce the filesytem.
> That only happens if the log worker is still running when we call
> xfs_quiesce_attr(). The log worker is cancelled at the end of
> xfs_quiesce_attr() by calling xfs_log_quiesce(), so just call it
> early here and then we can remove the counter altogether.
>
> Concurrent create, 50 million inodes, identical 16p/16GB virtual
> machines on different physical hosts. Machine A has twice the CPU
> cores per socket of machine B:
>
> unpatched patched
> machine A: 3m16s 2m00s
> machine B: 4m04s 4m05s
>
> Create rates:
> unpatched patched
> machine A: 282k+/-31k 468k+/-21k
> machine B: 231k+/-8k 233k+/-11k
>
> Concurrent rm of same 50 million inodes:
>
> unpatched patched
> machine A: 6m42s 2m33s
> machine B: 4m47s 4m47s
>
> The transaction rate on the fast machine went from just under
> 300k/sec to 700k/sec, which indicates just how much of a bottleneck
> this atomic counter was.
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
/me kinda wonders why removing the counter entirely has so little effect
on machine B, but seeing as I've been pondering killing this counter
myself for years,
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
--D
> ---
> fs/xfs/xfs_mount.h | 1 -
> fs/xfs/xfs_super.c | 17 +++++------------
> fs/xfs/xfs_trans.c | 27 +++++++++++----------------
> 3 files changed, 16 insertions(+), 29 deletions(-)
>
> diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> index c1f92c1847bb2..3725d25ad97e8 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -176,7 +176,6 @@ typedef struct xfs_mount {
> uint64_t m_resblks; /* total reserved blocks */
> uint64_t m_resblks_avail;/* available reserved blocks */
> uint64_t m_resblks_save; /* reserved blks @ remount,ro */
> - atomic_t m_active_trans; /* number trans frozen */
> struct delayed_work m_reclaim_work; /* background inode reclaim */
> struct delayed_work m_eofblocks_work; /* background eof blocks
> trimming */
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index aae469f73efeb..fa58cb07c8fdf 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -874,8 +874,10 @@ xfs_restore_resvblks(struct xfs_mount *mp)
> * there is no log replay required to write the inodes to disk - this is the
> * primary difference between a sync and a quiesce.
> *
> - * Note: xfs_log_quiesce() stops background log work - the callers must ensure
> - * it is started again when appropriate.
> + * We cancel log work early here to ensure all transactions the log worker may
> + * run have finished before we clean up and log the superblock and write an
> + * unmount record. The unfreeze process is responsible for restarting the log
> + * worker correctly.
> */
> void
> xfs_quiesce_attr(
> @@ -883,9 +885,7 @@ xfs_quiesce_attr(
> {
> int error = 0;
>
> - /* wait for all modifications to complete */
> - while (atomic_read(&mp->m_active_trans) > 0)
> - delay(100);
> + cancel_delayed_work_sync(&mp->m_log->l_work);
>
> /* force the log to unpin objects from the now complete transactions */
> xfs_log_force(mp, XFS_LOG_SYNC);
> @@ -899,12 +899,6 @@ xfs_quiesce_attr(
> if (error)
> xfs_warn(mp, "xfs_attr_quiesce: failed to log sb changes. "
> "Frozen image may not be consistent.");
> - /*
> - * Just warn here till VFS can correctly support
> - * read-only remount without racing.
> - */
> - WARN_ON(atomic_read(&mp->m_active_trans) != 0);
> -
> xfs_log_quiesce(mp);
> }
>
> @@ -1793,7 +1787,6 @@ static int xfs_init_fs_context(
> INIT_RADIX_TREE(&mp->m_perag_tree, GFP_ATOMIC);
> spin_lock_init(&mp->m_perag_lock);
> mutex_init(&mp->m_growlock);
> - atomic_set(&mp->m_active_trans, 0);
> INIT_WORK(&mp->m_flush_inodes_work, xfs_flush_inodes_worker);
> INIT_DELAYED_WORK(&mp->m_reclaim_work, xfs_reclaim_worker);
> INIT_DELAYED_WORK(&mp->m_eofblocks_work, xfs_eofblocks_worker);
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index b055a5ab53465..217937d743dbb 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -68,7 +68,6 @@ xfs_trans_free(
> xfs_extent_busy_clear(tp->t_mountp, &tp->t_busy, false);
>
> trace_xfs_trans_free(tp, _RET_IP_);
> - atomic_dec(&tp->t_mountp->m_active_trans);
> if (!(tp->t_flags & XFS_TRANS_NO_WRITECOUNT))
> sb_end_intwrite(tp->t_mountp->m_super);
> xfs_trans_free_dqinfo(tp);
> @@ -125,8 +124,6 @@ xfs_trans_dup(
> xfs_defer_move(ntp, tp);
>
> xfs_trans_dup_dqinfo(tp, ntp);
> -
> - atomic_inc(&tp->t_mountp->m_active_trans);
> return ntp;
> }
>
> @@ -275,7 +272,6 @@ xfs_trans_alloc(
> */
> WARN_ON(resp->tr_logres > 0 &&
> mp->m_super->s_writers.frozen == SB_FREEZE_COMPLETE);
> - atomic_inc(&mp->m_active_trans);
>
> tp->t_magic = XFS_TRANS_HEADER_MAGIC;
> tp->t_flags = flags;
> @@ -299,20 +295,19 @@ xfs_trans_alloc(
>
> /*
> * Create an empty transaction with no reservation. This is a defensive
> - * mechanism for routines that query metadata without actually modifying
> - * them -- if the metadata being queried is somehow cross-linked (think a
> - * btree block pointer that points higher in the tree), we risk deadlock.
> - * However, blocks grabbed as part of a transaction can be re-grabbed.
> - * The verifiers will notice the corrupt block and the operation will fail
> - * back to userspace without deadlocking.
> + * mechanism for routines that query metadata without actually modifying them --
> + * if the metadata being queried is somehow cross-linked (think a btree block
> + * pointer that points higher in the tree), we risk deadlock. However, blocks
> + * grabbed as part of a transaction can be re-grabbed. The verifiers will
> + * notice the corrupt block and the operation will fail back to userspace
> + * without deadlocking.
> *
> - * Note the zero-length reservation; this transaction MUST be cancelled
> - * without any dirty data.
> + * Note the zero-length reservation; this transaction MUST be cancelled without
> + * any dirty data.
> *
> - * Callers should obtain freeze protection to avoid two conflicts with fs
> - * freezing: (1) having active transactions trip the m_active_trans ASSERTs;
> - * and (2) grabbing buffers at the same time that freeze is trying to drain
> - * the buffer LRU list.
> + * Callers should obtain freeze protection to avoid a conflict with fs freezing
> + * where we can be grabbing buffers at the same time that freeze is trying to
> + * drain the buffer LRU list.
> */
> int
> xfs_trans_alloc_empty(
> --
> 2.26.2.761.g0e0b3e54be
>
