From f8d2f076d5fa6a91d97b26d1e83c98cff4630831 Mon Sep 17 00:00:00 2001 From: NeilBrown Date: Fri, 30 Nov 2018 10:33:18 +1100 Subject: [PATCH] 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 (from general rcu perspective) Signed-off-by: NeilBrown Signed-off-by: Al Viro Signed-off-by: Ruchit --- fs/namespace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index ed9a69dd82ad..caa455aeec1b 100755 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1870,7 +1870,7 @@ static void namespace_unlock(void) if (likely(hlist_empty(&head))) return; - synchronize_rcu(); + synchronize_rcu_expedited(); group_pin_kill(&head); }