xfs: reduce free inode accounting overhead

Shaokun Zhang reported that XFS was using substantial CPU time in
percpu_count_sum() when running a single threaded benchmark on
a high CPU count (128p) machine from xfs_mod_ifree(). The issue
is that the filesystem is empty when the benchmark runs, so inode
allocation is running with a very low inode free count.

With the percpu counter batching, this means comparisons when the
counter is less that 128 * 256 = 32768 use the slow path of adding
up all the counters across the CPUs, and this is expensive on high
CPU count machines.

The summing in xfs_mod_ifree() is only used to fire an assert if an
underrun occurs. The error is ignored by the higher level code.
Hence this is really just debug code and we don't need to run it
on production kernels, nor do we need such debug checks to return
error values just to trigger an assert.

Finally, xfs_mod_icount/xfs_mod_ifree are only called from
xfs_trans_unreserve_and_mod_sb(), so get rid of them and just
directly call the percpu_counter_add/percpu_counter_compare
functions. The compare functions are now run only on debug builds as
they are internal to ASSERT() checks and so only compiled in when
ASSERTs are active (CONFIG_XFS_DEBUG=y or CONFIG_XFS_WARN=y).

Reported-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 0 insertions, 33 deletions

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index bb91f04266b9..d5dcf9869860 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1190,39 +1190,6 @@ xfs_log_sbcount(xfs_mount_t *mp)
 }
 
 /*
- * Deltas for the inode count are +/-64, hence we use a large batch size
- * of 128 so we don't need to take the counter lock on every update.
- */
-#define XFS_ICOUNT_BATCH	128
-int
-xfs_mod_icount(
-	struct xfs_mount	*mp,
-	int64_t			delta)
-{
-	percpu_counter_add_batch(&mp->m_icount, delta, XFS_ICOUNT_BATCH);
-	if (__percpu_counter_compare(&mp->m_icount, 0, XFS_ICOUNT_BATCH) < 0) {
-		ASSERT(0);
-		percpu_counter_add(&mp->m_icount, -delta);
-		return -EINVAL;
-	}
-	return 0;
-}
-
-int
-xfs_mod_ifree(
-	struct xfs_mount	*mp,
-	int64_t			delta)
-{
-	percpu_counter_add(&mp->m_ifree, delta);
-	if (percpu_counter_compare(&mp->m_ifree, 0) < 0) {
-		ASSERT(0);
-		percpu_counter_add(&mp->m_ifree, -delta);
-		return -EINVAL;
-	}
-	return 0;
-}
-
-/*
  * Deltas for the block count can vary from 1 to very large, but lock contention
  * only occurs on frequent small block count updates such as in the delayed
  * allocation path for buffered writes (page a time updates). Hence we set