libfs: Add a lock class for the offset map's xa_lock

Commit Message

Chuck Lever July 24, 2023, 2:43 p.m. UTC

From: Chuck Lever <[e-mail]>

Tie the dynamically-allocated xarray locks into a single class so
contention on the directory offset xarrays can be observed.

Signed-off-by: Chuck Lever <[e-mail]>
---
 fs/libfs.c |    3 +++
 1 file changed, 3 insertions(+)

I've been looking into the recent kernel bot reports of performance
regressions on the will-it-scale benchmark.

https://lore.kernel.org/linux-mm/[e-mail]/

I haven't been able to run the reproducer yet, but I have created a
small change to demonstrate that it is unlikely that it is the
xa_lock itself that is the issue. All tests I've run here show "0.0"
in the lock_stat contention metrics for the simple_offset_xa_lock
class.

It seems reasonable to include this small change in the patches
already applied to your tree.

Comments

Christian Brauner July 24, 2023, 2:55 p.m. UTC | #1

On Mon, 24 Jul 2023 10:43:57 -0400, Chuck Lever wrote:
> Tie the dynamically-allocated xarray locks into a single class so
> contention on the directory offset xarrays can be observed.
> 
> 

Applied to the vfs.readdir branch of the vfs/vfs.git tree.
Patches in the vfs.readdir branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.readdir

[1/1] libfs: Add a lock class for the offset map's xa_lock
      https://git.kernel.org/vfs/vfs/c/fc66c4c9dc90

Patch

diff --git a/fs/libfs.c b/fs/libfs.c
index 68b0000dc518..fcc0f1f3c2dc 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -249,6 +249,8 @@  static unsigned long dentry2offset(struct dentry *dentry)
 	return (unsigned long)dentry->d_fsdata;
 }
 
+static struct lock_class_key simple_offset_xa_lock;
+
 /**
  * simple_offset_init - initialize an offset_ctx
  * @octx: directory offset map to be initialized
@@ -257,6 +259,7 @@  static unsigned long dentry2offset(struct dentry *dentry)
 void simple_offset_init(struct offset_ctx *octx)
 {
 	xa_init_flags(&octx->xa, XA_FLAGS_ALLOC1);
+	lockdep_set_class(&octx->xa.xa_lock, &simple_offset_xa_lock);
 
 	/* 0 is '.', 1 is '..', so always start with offset 2 */
 	octx->next_offset = 2;