[rh7] ms/VFS: use synchronize_rcu_expedited() in namespace_unlock()

Submitted by Andrey Ryabinin on March 4, 2020, 2:33 p.m.


Message ID 20200304143325.22607-1-aryabinin@virtuozzo.com
State New
Series "ms/VFS: use synchronize_rcu_expedited() in namespace_unlock()"
Headers show

Commit Message

Andrey Ryabinin March 4, 2020, 2:33 p.m.
From: NeilBrown <neilb@suse.com>

The synchronize_rcu() in namespace_unlock() is called every time
a filesystem is unmounted.  If a great many filesystems are mounted,
this can cause a noticable slow-down in, for example, system shutdown.

The sequence:
  mkdir -p /tmp/Mtest/{0..5000}
  time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
  time umount /tmp/Mtest/*

on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
100 seconds to unmount them.

Boot the same VM with 1 CPU and it takes 18 seconds to mount the
tmpfs filesystems, but only 36 to unmount.

If we change the synchronize_rcu() to synchronize_rcu_expedited()
the umount time on a 4-cpu VM drop to 0.6 seconds

I think this 200-fold speed up is worth the slightly high system
impact of using synchronize_rcu_expedited().

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> (from general rcu perspective)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

(cherry-picked from commit 22cb7405fada5305926d9bbcb476c1cd638d2dab)
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
 fs/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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diff --git a/fs/namespace.c b/fs/namespace.c
index d4527f5bd75a..1890491ac9e8 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1523,7 +1523,7 @@  static void namespace_unlock(void)
 	if (likely(hlist_empty(&head)))
-	synchronize_rcu();
+	synchronize_rcu_expedited();