From: NeilBrown <neilb@suse.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: [PATCH 1/3] VFS: use synchronize_rcu_expedited() in namespace_unlock()
Date: Thu, 09 Nov 2017 14:22:07 +1100 [thread overview]
Message-ID: <151019772756.30101.64944229146330338.stgit@noble> (raw)
In-Reply-To: <151019756744.30101.3832608128627682973.stgit@noble>
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@linux.vnet.ibm.com> (from general rcu perspective)
Signed-off-by: NeilBrown <neilb@suse.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);
}
next prev parent reply other threads:[~2017-11-09 3:25 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-09 3:22 [PATCH 0/3] Three VFS patch resends NeilBrown
2017-11-09 3:22 ` NeilBrown [this message]
2017-11-09 3:22 ` [PATCH 2/3] Improve fairness when locking the per-superblock s_anon list NeilBrown
2017-11-09 19:52 ` Linus Torvalds
2017-11-09 20:50 ` Al Viro
2017-11-09 23:09 ` NeilBrown
2017-11-09 23:19 ` Al Viro
2017-11-10 0:02 ` Linus Torvalds
2017-11-10 8:50 ` Christoph Hellwig
2017-11-09 3:22 ` [PATCH 3/3] VFS: close race between getcwd() and d_move() NeilBrown
2017-11-09 11:41 ` Nikolay Borisov
2017-11-09 13:08 ` Matthew Wilcox
2017-11-09 16:02 ` Nikolay Borisov
2017-11-09 20:23 ` Linus Torvalds
2017-11-09 22:14 ` NeilBrown
2017-11-10 1:40 ` Linus Torvalds
2017-11-10 4:45 ` NeilBrown
2017-11-10 19:52 ` Linus Torvalds
2017-11-10 20:53 ` Al Viro
2017-11-21 23:50 ` Al Viro
2017-11-22 1:31 ` NeilBrown
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