From: NeilBrown <neilb@suse.com>
Subject: VFS: use synchronize_rcu_expedited() in namespace_unlock()
References: bsc#1024412
Patch-mainline: submitted, https://patchwork.kernel.org/patch/10027405/
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 is 8 seconds to mount and 0.6 to
unmount.
I think this 200-fold speed up is worth the slightly higher system
impact of use synchronize_rcu_expedited().
Signed-off-by: NeilBrown <neilb@suse.com>
Acked-by: NeilBrown <neilb@suse.com>
(Paul McKenney acked the patch, Al Viro hasn't responded yet).
---
fs/namespace.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1370,7 +1370,7 @@ static void namespace_unlock(void)
if (likely(hlist_empty(&head)))
return;
- synchronize_rcu();
+ synchronize_rcu_expedited();
group_pin_kill(&head);
}