Messages in this thread Patch in this message | | | From | NeilBrown <> | Date | Fri, 30 Nov 2018 10:33:18 +1100 | Subject | Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock() |
| |
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> ---
Al Viro says "I can live with this one" but this still hasn't landed. Maybe someone else could apply it?
Thanks, NeilBrown
Full quote from Al on 5th Oct: > Umm... IIRC, the last one got sidetracked on the other thing in the series... > <checks> that was s_anon stuff. I can live with this one; FWIW, what kind > of load would trigger the impact of the change? Paul? which Paul replied to.
fs/namespace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/namespace.c b/fs/namespace.c index a7f91265ea67..43a0d2c7449d 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1360,7 +1360,7 @@ static void namespace_unlock(void) if (likely(hlist_empty(&head))) return; - synchronize_rcu(); + synchronize_rcu_expedited(); group_pin_kill(&head); } -- 2.14.0.rc0.dirty[unhandled content-type:application/pgp-signature]
| |