Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()

From: Al Viro <viro@zeniv.linux.org.uk>
To: NeilBrown <neilb@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	paulmck@linux.vnet.ibm.com, Florian Weimer <fweimer@redhat.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Josh Triplett <josh@joshtriplett.org>
Subject: Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()
Date: Thu, 29 Nov 2018 23:52:22 +0000	[thread overview]
Message-ID: <20181129235222.GJ2217@ZenIV.linux.org.uk> (raw)
In-Reply-To: <87mupro3wh.fsf@notabene.neil.brown.name>

On Fri, Nov 30, 2018 at 10:33:18AM +1100, NeilBrown wrote:
> 
> 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 <paulmck@linux.vnet.ibm.com> (from general rcu perspective)
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
> 
> Al Viro says "I can live with this one" but this still hasn't landed.
> Maybe someone else could apply it?

Applied (in work.misc, once I push it out)