From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753318Ab0KYO62 (ORCPT ); Thu, 25 Nov 2010 09:58:28 -0500 Received: from e6.ny.us.ibm.com ([32.97.182.146]:40336 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752219Ab0KYO61 (ORCPT ); Thu, 25 Nov 2010 09:58:27 -0500 Date: Thu, 25 Nov 2010 06:58:24 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Lai Jiangshan , LKML , Thomas Gleixner , Peter Zijlstra , Steven Rostedt , Ingo Molnar Subject: Re: [PATCH 0/2] rcu: Fix series of spurious RCU softirqs Message-ID: <20101125145824.GD2162@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1290558673-23580-1-git-send-crap-fweisbec@gmail.com> <4CEDDB2A.2020807@cn.fujitsu.com> <20101125073857.GB2538@nowhere> <4CEE1FD2.7040707@cn.fujitsu.com> <20101125092706.GH2538@nowhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101125092706.GH2538@nowhere> User-Agent: Mutt/1.5.20 (2009-06-14) X-Content-Scanned: Fidelis XPS MAILER Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 25, 2010 at 10:27:08AM +0100, Frederic Weisbecker wrote: > On Thu, Nov 25, 2010 at 04:35:30PM +0800, Lai Jiangshan wrote: > > On 11/25/2010 03:38 PM, Frederic Weisbecker wrote: > > > On Thu, Nov 25, 2010 at 11:42:34AM +0800, Lai Jiangshan wrote: > > >> On 11/24/2010 08:31 AM, Frederic Weisbecker wrote: > > >>> Hi, > > >>> > > >>> I've observed some not so unfrequent series of spurious rcu > > >>> softirqs, sometimes happening at each ticks for a random > > >>> while. > > >>> > > >>> These patches aims at fixing them. > > >>> > > >>> Thanks. > > >>> > > >>> Frederic Weisbecker (2): > > >>> rcu: Don't chase unnecessary quiescent states after extended grace periods > > >>> rcu: Stop checking quiescent states after grace period completion from remote > > >>> > > >> > > >> If we ensure rdp->gpnum >= rdp->completed is always true, the problems as > > >> you described will not be existed. Or maybe I misunderstand you. > > >> > > >> rdp->gpnum >= rdp->completed is a very important guarantee I think. > > >> (In my RCURING, it is guaranteed.) I'm afraid there are some other > > >> problems still hidden if it is not guaranteed. > > >> > > >> so I recommend: (code is better than words) > > >> > > >> diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > >> index d5bc439..af4e87a 100644 > > >> --- a/kernel/rcutree.c > > >> +++ b/kernel/rcutree.c > > >> @@ -648,6 +648,13 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat > > >> > > >> /* Remember that we saw this grace-period completion. */ > > >> rdp->completed = rnp->completed; > > >> + > > >> + /* Ensure ->gpnum >= ->completed after NO_HZ */ > > >> + if (unlikely(rnp->completed - rdp->gpnum > 0 > > >> + || rdp->gpnum - rnp->gpnum > 0)) { > > >> + rdp->gpnum = rnp->completed; > > >> + rdp->qs_pending = 0; > > > > > > > > > That's an alternative to my first patch yeah. > > > > Since rdp->gpnum >= rdp->completed is guaranteed. > > your second patch is not needed, the problem is also fixed. > > > > if rnp->gpnum == rnp->completed, rcu_report_qs_rdp() will not be called. > > it is because rdp->qs_pending == 0 when rnp->gpnum == rnp->completed. > > > Aaah... > > > > > > And if rdp->gpnum >= rdp->completed > > > must be a guarantee outside the rnp lock, then it's certainly better because > > > the lock is relaxed between rcu_process_gp_end() and note_new_gpnum(), and > > > both values are async in this lockless frame. > > > > > > But perhaps this shouldn't touch rdp->qs_pending: > > > > if rdp->gpnum == rnp->completed, it means we don't need a qs for rdp->gpnum, > > it is completed. so we must set rdp->qs_pending = 0; > > > > when we really need a qs, rdp->qs_pending will be fixed in note_new_gp_new(). > > > Ok that makes all sense now! > > I'm just not sure about your check above. > > (rdp->gpnum - rnp->gpnum > 0) can never happen, right? > > Also perhaps we should set rdp->qs_pending = 0 only if > rnp->completed == rdp->completed? > > Which in the end would be: > > /* Remember that we saw this grace-period completion. */ > rdp->completed = rnp->completed; > > + if (rdp->gpnum < rdp->completed) > + rdp->gpnum = rdp->completed; > + > + if (rdp->gpnum == rdp->completed) > + rdp->qs_pending = 0; > > > And then if there is a new grace period to handle, this will > be done through note_new_pgnum(). > > Hm? Given that it is Thanksgiving holiday here in USA, I am going to give you guys a several days to come to agreement on this. Then I will inspect the resulting patch. ;-) Thanx, Paul