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VFS: use synchronize_rcu_expedited() in namespace_unlock()

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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>
This commit is contained in:
NeilBrown 2018-11-30 10:33:18 +11:00 committed by Gagan Malvi
parent 2b8f19d665
commit a1ac400e39
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1 changed files with 1 additions and 1 deletions
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2
fs/namespace.c
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@ -1378,7 +1378,7 @@ static void namespace_unlock(void)
if (likely(hlist_empty(&head))) if (likely(hlist_empty(&head)))
return; return;
synchronize_rcu(); synchronize_rcu_expedited();
group_pin_kill(&head); group_pin_kill(&head);
} }