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Date: Thu, 09 Nov 2017 14:22:07 +1100 From: NeilBrown <neilb@...e.com> To: Al Viro <viro@...iv.linux.org.uk> Cc: linux-fsdevel@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>, linux-kernel@...r.kernel.org Subject: [PATCH 1/3] 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 higher system impact of using synchronize_rcu_expedited(). Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> (from general rcu perspective) Signed-off-by: NeilBrown <neilb@...e.com> --- fs/namespace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index d18deb4c410b..23cdf6c62895 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1420,7 +1420,7 @@ static void namespace_unlock(void) if (likely(hlist_empty(&head))) return; - synchronize_rcu(); + synchronize_rcu_expedited(); group_pin_kill(&head); }
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