Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755751Ab3FDP3n (ORCPT ); Tue, 4 Jun 2013 11:29:43 -0400 Received: from mail-oa0-f51.google.com ([209.85.219.51]:52197 "EHLO mail-oa0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755554Ab3FDP3k (ORCPT ); Tue, 4 Jun 2013 11:29:40 -0400 MIME-Version: 1.0 In-Reply-To: <20130604144451.GJ14973@somewhere> References: <1369927385-7801-1-git-send-email-vincent.guittot@linaro.org> <20130603224836.GA9388@somewhere> <20130604093611.GJ8923@twins.programming.kicks-ass.net> <20130604102620.GB14012@somewhere> <20130604111900.GB14973@somewhere> <20130604144451.GJ14973@somewhere> Date: Tue, 4 Jun 2013 17:29:39 +0200 Message-ID: Subject: Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK From: Vincent Guittot To: Frederic Weisbecker Cc: Peter Zijlstra , linux-kernel , "linaro-kernel@lists.linaro.org" , Ingo Molnar Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3773 Lines: 87 On 4 June 2013 16:44, Frederic Weisbecker wrote: > On Tue, Jun 04, 2013 at 01:48:47PM +0200, Vincent Guittot wrote: >> On 4 June 2013 13:19, Frederic Weisbecker wrote: >> > On Tue, Jun 04, 2013 at 01:11:47PM +0200, Vincent Guittot wrote: >> >> On 4 June 2013 12:26, Frederic Weisbecker wrote: >> >> > On Tue, Jun 04, 2013 at 11:36:11AM +0200, Peter Zijlstra wrote: >> >> >> >> >> >> The best I can seem to come up with is something like the below; but I think >> >> >> its ghastly. Surely we can do something saner with that bit. >> >> >> >> >> >> Having to clear it at 3 different places is just wrong. >> >> > >> >> > We could clear the flag early in scheduler_ipi() and set some >> >> > specific value in rq->idle_balance that tells we want nohz idle >> >> > balancing from the softirq, something like this untested: >> >> >> >> I'm not sure that we can have less than 2 places to clear it: cancel >> >> place or acknowledge place otherwise we can face a situation where >> >> idle load balance will be triggered 2 consecutive times because >> >> NOHZ_BALANCE_KICK will be cleared before the idle load balance has >> >> been done and had a chance to migrate tasks. >> > >> > I guess it depends what is the minimum value of rq->next_balance, it seems >> > to be large enough to avoid this kind of incident. Although I don't >> > know well the whole logic with rq->next_balance and ilb trigger so I must >> > defer to you. >> >> In the trace that was showing the issue, i can see that both CPU0 and >> CPU1 were trying to trig ILB almost simultaneously and the >> test_and_set NOHZ_BALANCE_KICK filters one request so i would say that >> clearing the bit before the end of the idle load balance sequence can >> generate such sequence > > I see. > >> >> In the sequence below, i have minimized the clear of NOHZ_BALANCE_KICK >> in 2 places : acknowledge and cancel. I have reused part of the >> proposal from peter which clears the bit if the condition doesn't >> match but i have reordered the tests to done that only if all other >> condition are matching >> >> static inline bool got_nohz_idle_kick(void) >> { >> - int cpu = smp_processor_id(); >> - return idle_cpu(cpu) && test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu)); >> + bool nohz_kick = test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu)); >> + >> + if (!nohz_kick) >> + return false; >> + >> + if (idle_cpu(cpu) && !need_resched()) >> + return true; >> + >> + clear_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu)); >> + return false; >> } >> >> #else /* CONFIG_NO_HZ_COMMON */ >> @@ -1393,8 +1401,9 @@ static void sched_ttwu_pending(void) >> >> void scheduler_ipi(void) >> { >> - if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick() >> - && !tick_nohz_full_cpu(smp_processor_id())) >> + if (llist_empty(&this_rq()->wake_list) >> + && !tick_nohz_full_cpu(smp_processor_id()) >> + && !got_nohz_idle_kick()) >> return; > > But we still need got_nohz_idle_kick() to be the first check, don't we? Otherwise > if we run an "idle -> quick task slice -> idle" sequence we may keep the flag > but lose the notifying IPI in between. I'm not sure to catch the sequence you are describing above: "idle -> quick task slice -> idle". In addition, got_nohz_idle_kick must be the last tested condition (in my proposal) in order to clear NOHZ_BALANCE_KICK only if we are sure that we are going to return without possibility to trig the Idle load balance Vincent -- 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/