From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757170Ab2IMK3d (ORCPT ); Thu, 13 Sep 2012 06:29:33 -0400 Received: from mailout-de.gmx.net ([213.165.64.23]:37374 "HELO mailout-de.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1751708Ab2IMK3b (ORCPT ); Thu, 13 Sep 2012 06:29:31 -0400 X-Authenticated: #14349625 X-Provags-ID: V01U2FsdGVkX1//FQ0RvB3/lcfDSCh+RcoAXRtNCTnKP8AG009OAO Sv4t5/bfsCQfp+ Message-ID: <1347532167.6821.75.camel@marge.simpson.net> Subject: Re: [RFC] sched: nohz_idle_balance From: Mike Galbraith To: Peter Zijlstra Cc: Vincent Guittot , linux-kernel@vger.kernel.org, linaro-dev@lists.linaro.org, mingo@redhat.com Date: Thu, 13 Sep 2012 12:29:27 +0200 In-Reply-To: <1347524381.15764.100.camel@twins> References: <1347509486-8688-1-git-send-email-vincent.guittot@linaro.org> <1347518991.6821.45.camel@marge.simpson.net> <1347524381.15764.100.camel@twins> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 X-Y-GMX-Trusted: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2012-09-13 at 10:19 +0200, Peter Zijlstra wrote: > On Thu, 2012-09-13 at 08:49 +0200, Mike Galbraith wrote: > > On Thu, 2012-09-13 at 06:11 +0200, Vincent Guittot wrote: > > > On tickless system, one CPU runs load balance for all idle CPUs. > > > The cpu_load of this CPU is updated before starting the load balance > > > of each other idle CPUs. We should instead update the cpu_load of the balance_cpu. > > > > > > Signed-off-by: Vincent Guittot > > > --- > > > kernel/sched/fair.c | 11 ++++++----- > > > 1 file changed, 6 insertions(+), 5 deletions(-) > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > index 1ca4fe4..9ae3a5b 100644 > > > --- a/kernel/sched/fair.c > > > +++ b/kernel/sched/fair.c > > > @@ -4794,14 +4794,15 @@ static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle) > > > if (need_resched()) > > > break; > > > > > > - raw_spin_lock_irq(&this_rq->lock); > > > - update_rq_clock(this_rq); > > > - update_idle_cpu_load(this_rq); > > > - raw_spin_unlock_irq(&this_rq->lock); > > > + rq = cpu_rq(balance_cpu); > > > + > > > + raw_spin_lock_irq(&rq->lock); > > > + update_rq_clock(rq); > > > + update_idle_cpu_load(rq); > > > + raw_spin_unlock_irq(&rq->lock); > > > > > > rebalance_domains(balance_cpu, CPU_IDLE); > > > > > > - rq = cpu_rq(balance_cpu); > > > if (time_after(this_rq->next_balance, rq->next_balance)) > > > this_rq->next_balance = rq->next_balance; > > > } > > > > Ew, banging locks and updating clocks to what good end? > > Well, updating the load statistics on the cpu you're going to balance > seems like a good end to me.. ;-) No point updating the local statistics > N times and leaving the ones you're going to balance stale for god knows > how long. Sure, the goal is fine, but I wonder about the price vs payoff. I was thinking perhaps the redundant updates should go away instead, unless stats are shown to be causing real world pain. -Mike