[PATCH 04/18] xfs: lockless per-ag lookups

[Alex Elder]

On Tue, 2010-09-14 at 20:56 +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> When we start taking a reference to the per-ag for every cached
> buffer in the system, kernel lockstat profiling on an 8-way create
> workload shows the mp->m_perag_lock has higher acquisition rates
> than the inode lock and has significantly more contention. That is,
> it becomes the highest contended lock in the system.
> 
> The perag lookup is trivial to convert to lock-less RCU lookups
> because perag structures never go away. Hence the only thing we need
> to protect against is tree structure changes during a grow. THis can
> be done simply by replacing the locking in xfs_perag_get() with RCU
> read locking. This removes the mp->m_perag_lock completely from this
> path.

Nice.

Reviewed-by: Alex Elder <aelder at sgi.com>

> Signed-off-by: Dave Chinner <dchinner at redhat.com>
> ---
>  fs/xfs/xfs_ag.h    |    3 +++
>  fs/xfs/xfs_mount.c |   25 +++++++++++++++++--------
>  2 files changed, 20 insertions(+), 8 deletions(-)