Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751772AbaBKLLz (ORCPT ); Tue, 11 Feb 2014 06:11:55 -0500 Received: from mail-we0-f172.google.com ([74.125.82.172]:51615 "EHLO mail-we0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750809AbaBKLLy (ORCPT ); Tue, 11 Feb 2014 06:11:54 -0500 Message-ID: <52FA0577.6080607@linaro.org> Date: Tue, 11 Feb 2014 12:11:51 +0100 From: Daniel Lezcano User-Agent: Mozilla/5.0 (X11; Linux i686; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 To: Preeti Murthy , Peter Zijlstra CC: mingo@kernel.org, alex.shi@linaro.org, LKML , Lists linaro-kernel , Preeti U Murthy Subject: Re: [PATCH V2 2/3] sched: Fix race in idle_balance() References: <1391728237-4441-1-git-send-email-daniel.lezcano@linaro.org> <1391728237-4441-3-git-send-email-daniel.lezcano@linaro.org> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/10/2014 10:24 AM, Preeti Murthy wrote: > HI Daniel, > > Isn't the only scenario where another cpu can put an idle task on > our runqueue, Well, I am not sure to understand what you meant, but I assume you are asking if it is possible to have a task to be pulled when we are idle, right ? This patch fixes the race when the current cpu is *about* to enter idle when calling schedule(). > in nohz_idle_balance() where only the cpus in > the nohz.idle_cpus_mask are iterated through. But for the case > that this patch is addressing, the cpu in question is not yet a part > of the nohz.idle_cpus_mask right? > > Any other case would trigger load balancing on the same cpu, but > we are preempt_disabled and interrupt disabled at this point. > > Thanks > > Regards > Preeti U Murthy > > On Fri, Feb 7, 2014 at 4:40 AM, Daniel Lezcano > wrote: >> The scheduler main function 'schedule()' checks if there are no more tasks >> on the runqueue. Then it checks if a task should be pulled in the current >> runqueue in idle_balance() assuming it will go to idle otherwise. >> >> But the idle_balance() releases the rq->lock in order to lookup in the sched >> domains and takes the lock again right after. That opens a window where >> another cpu may put a task in our runqueue, so we won't go to idle but >> we have filled the idle_stamp, thinking we will. >> >> This patch closes the window by checking if the runqueue has been modified >> but without pulling a task after taking the lock again, so we won't go to idle >> right after in the __schedule() function. >> >> Cc: alex.shi@linaro.org >> Cc: peterz@infradead.org >> Cc: mingo@kernel.org >> Signed-off-by: Daniel Lezcano >> Signed-off-by: Peter Zijlstra >> --- >> kernel/sched/fair.c | 7 +++++++ >> 1 file changed, 7 insertions(+) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 428bc9d..5ebc681 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -6589,6 +6589,13 @@ void idle_balance(struct rq *this_rq) >> >> raw_spin_lock(&this_rq->lock); >> >> + /* >> + * While browsing the domains, we released the rq lock. >> + * A task could have be enqueued in the meantime >> + */ >> + if (this_rq->nr_running && !pulled_task) >> + return; >> + >> if (pulled_task || time_after(jiffies, this_rq->next_balance)) { >> /* >> * We are going idle. next_balance may be set based on >> -- >> 1.7.9.5 >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/