kernfs: allocate the open node outside the open file mutex

kernfs_get_open_node() allocates the kernfs_open_node while holding one
of the hashed kernfs_open_file mutexes, so opening a file nobody has open
yet can enter reclaim with that mutex held.  The mutex is shared by every
node that hashes to it, so unrelated opens, closes and xattr updates in
the same bucket wait.

It is the most contended lock in kernfs on our fleet: 2.19M waiters over
29 days, more than two hundred times the waiters on kernfs_rwsem, and the
highest median hold of any kernfs lock at 77ms.  Most of it is monitoring
daemons opening cgroup control files, and the open node is freed once the
last descriptor closes, so an open-read-close loop allocates every time.

Look at kn->attr.open before taking the mutex, and allocate then if
nothing has the file open.  Should the peek be wrong, which needs the
last descriptor to close inside the window, fall back to allocating under
the mutex as before.

Eight tasks opening and closing four cgroup files 20000 times each, with
lock_stat on the hashed mutex:

                     contentions    hold total    hold avg
   before                    198        4.19 s     8.67 us
   after                     119        2.31 s     4.78 us

and the same number of acquisitions either way.

This does not empty the bucket.  kernfs_fop_release() and
kernfs_xattr_set() still sleep under the same mutex.  It stops opens
doing it.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://patch.msgid.link/20260910045433.489413-1-shakeel.butt@linux.dev
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions