From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:45640 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932348AbdJZNuT (ORCPT ); Thu, 26 Oct 2017 09:50:19 -0400 Received: from pps.filterd (m0098393.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v9QDlnCK116037 for ; Thu, 26 Oct 2017 09:50:19 -0400 Received: from e19.ny.us.ibm.com (e19.ny.us.ibm.com [129.33.205.209]) by mx0a-001b2d01.pphosted.com with ESMTP id 2duf6e680s-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 26 Oct 2017 09:50:17 -0400 Received: from localhost by e19.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 26 Oct 2017 09:50:13 -0400 Date: Thu, 26 Oct 2017 06:50:09 -0700 From: "Paul E. McKenney" To: NeilBrown Cc: Alexander Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Josh Triplett Subject: Re: [PATCH] VFS: use synchronize_rcu_expedited() in namespace_unlock() Reply-To: paulmck@linux.vnet.ibm.com References: <87y3nyd4pu.fsf@notabene.neil.brown.name> <20171026122743.GX3659@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171026122743.GX3659@linux.vnet.ibm.com> Message-Id: <20171026135009.GA27806@linux.vnet.ibm.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Oct 26, 2017 at 05:27:43AM -0700, Paul E. McKenney wrote: > On Thu, Oct 26, 2017 at 01:26:37PM +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 is 8 seconds to mount and 0.6 to > > unmount. > > > > I think this 200-fold speed up is worth the slightly higher system > > impact of use synchronize_rcu_expedited(). > > > > Signed-off-by: NeilBrown > > --- > > > > Cc: to Paul and Josh in case they'll correct me if using _expedited() > > is really bad here. > > I suspect that filesystem unmount is pretty rare in production real-time > workloads, which are the ones that might care. So I would guess that > this is OK. > > If the real-time guys ever do want to do filesystem unmounts while their > real-time applications are running, they might modify this so that it can > use synchronize_rcu() instead for real-time builds of the kernel. Which they can already do using the rcupdate.rcu_normal boot parameter. Thanx, Paul > But just for completeness, one way to make this work across the board > might be to instead use call_rcu(), with the callback function kicking > off a workqueue handler to do the rest of the unmount. Of course, > in saying that, I am ignoring any mutexes that you might be holding > across this whole thing, and also ignoring any problems that might arise > when returning to userspace with some portion of the unmount operation > still pending. (For example, someone unmounting a filesystem and then > immediately remounting that same filesystem.) > > Thanx, Paul > > > Thanks, > > NeilBrown > > > > > > fs/namespace.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/fs/namespace.c b/fs/namespace.c > > index 3b601f115b6c..fce91c447fab 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); > > } > > -- > > 2.14.0.rc0.dirty > > > >