linux-kernel - [PATCH 1/3] VFS: use synchronize_rcu_expedited() in namespace

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Message-ID: <151019772756.30101.64944229146330338.stgit@noble>
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);
 }