From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Thu, 29 Nov 2018 23:52:22 +0000 From: Al Viro To: NeilBrown Cc: Andrew Morton , Linus Torvalds , paulmck@linux.vnet.ibm.com, Florian Weimer , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Josh Triplett Subject: Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock() Message-ID: <20181129235222.GJ2217@ZenIV.linux.org.uk> References: <87y3nyd4pu.fsf@notabene.neil.brown.name> <20171026122743.GX3659@linux.vnet.ibm.com> <20171127144125.GF3624@linux.vnet.ibm.com> <87induxd3u.fsf@notabene.neil.brown.name> <87tvm1rxme.fsf@notabene.neil.brown.name> <20181005014002.GS32577@ZenIV.linux.org.uk> <87mupro3wh.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87mupro3wh.fsf@notabene.neil.brown.name> Sender: linux-kernel-owner@vger.kernel.org List-ID: 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 (from general rcu perspective) > Signed-off-by: NeilBrown > --- > > 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)