From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754182Ab0IMRgw (ORCPT ); Mon, 13 Sep 2010 13:36:52 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:43653 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754163Ab0IMRgv (ORCPT ); Mon, 13 Sep 2010 13:36:51 -0400 Date: Mon, 13 Sep 2010 19:36:34 +0200 From: Ingo Molnar To: Mathieu Desnoyers Cc: Peter Zijlstra , LKML , Linus Torvalds , Andrew Morton , Steven Rostedt , Thomas Gleixner , Tony Lindgren , Mike Galbraith Subject: Re: [RFC PATCH] check_preempt_tick should not compare vruntime with wall time Message-ID: <20100913173634.GA17815@elte.hu> References: <20100911174003.051303123@efficios.com> <1284231470.2251.52.camel@laptop> <20100911195708.GA9273@Krystal> <1284288072.2251.91.camel@laptop> <20100912203712.GD32327@Krystal> <1284382387.2275.265.camel@laptop> <1284383758.2275.283.camel@laptop> <20100913135621.GA13442@Krystal> <1284387398.2275.311.camel@laptop> <20100913161641.GA28707@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100913161641.GA28707@Krystal> User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -2.0 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mathieu Desnoyers wrote: > * Peter Zijlstra (peterz@infradead.org) wrote: > > On Mon, 2010-09-13 at 09:56 -0400, Mathieu Desnoyers wrote: > [...] > > > > static void > > > > check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr) > > > > { > > > > - unsigned long ideal_runtime, delta_exec; > > > > + unsigned long slice = sched_slice(cfs_rq, curr); > > > > > > So you still compute the sched_slice(), based on sched_period(), based on > > > sysctl_sched_min_granularity *= nr_running when there are more than nr_latency > > > running threads. > > > > What's wrong with that? I keep asking you, you keep not giving an > > answer. Stop focussing on nr_latency, its an by produce not a > > fundamental entity. > > > > period := max(latency, min_gran * nr_running) > > > > See, no nr_latency -- the one and only purpose of nr_latency is avoiding > > that multiplication when possible. > > OK, the long IRC discussions we just had convinced me that the current > scheme takes things into account by adapting the granularity > dynamically, but also got me to notice that check_preempt seems to > compare vruntime with wall time, which is utterly incorrect. So maybe > all my patch was doing was to expose this bug: > > --- > kernel/sched_fair.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > Index: linux-2.6-lttng.git/kernel/sched_fair.c > =================================================================== > --- linux-2.6-lttng.git.orig/kernel/sched_fair.c > +++ linux-2.6-lttng.git/kernel/sched_fair.c > @@ -869,7 +869,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq > struct sched_entity *se = __pick_next_entity(cfs_rq); > s64 delta = curr->vruntime - se->vruntime; > > - if (delta > ideal_runtime) > + if (delta > calc_delta_fair(ideal_runtime, curr)) > resched_task(rq_of(cfs_rq)->curr); > } > } It should have no effect at all on your latency measurements, as calc_delta_fair() is a NOP for nice-0 tasks: static inline unsigned long calc_delta_fair(unsigned long delta, struct sched_entity *se) { if (unlikely(se->load.weight != NICE_0_LOAD)) delta = calc_delta_mine(delta, NICE_0_LOAD, &se->load); return delta; } Thanks, Ingo