Re: [PATCH 2/2] xfs: remove the m_active_trans counter

[Christoph Hellwig <hch@infradead.org>]

On Wed, May 20, 2020 at 08:23:10AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> 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@redhat.com>
> ---
>  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.
>   *
> + * 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);

Shouldn't the cancel_delayed_work_sync for l_work in xfs_log_quiesce
be removed now given that we've already cancelled it here?