From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755751Ab1LZPsr (ORCPT ); Mon, 26 Dec 2011 10:48:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:29718 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755430Ab1LZPsp (ORCPT ); Mon, 26 Dec 2011 10:48:45 -0500 Date: Mon, 26 Dec 2011 16:42:57 +0100 From: Oleg Nesterov To: Kirill Tkhai Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Kirill Tkhai Subject: Re: [PATCH]sched: stop hrtick timer if running task is switching from fair scheduling class to another Message-ID: <20111226154257.GA21445@redhat.com> References: <1324584060.4222.15.camel@hp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1324584060.4222.15.camel@hp> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/23, Kirill Tkhai wrote: > > [PATCH]sched: stop hrtick timer if running task is switching from fair > scheduling class to another > > We have to stop hrtick timer to avoid excess interrupt. Not-fair tasks > are not interested in fair's hrtick. RT class uses its own fixed > timeslice (in case of RR), which doesn't depend on current value of hrtick > timer. Well. I shouldn't try to comment this patch, I do not really understand this code. But since nobody else replies... > @@ -5271,6 +5271,13 @@ static void switched_from_fair(struct rq *rq, > struct task_struct *p) > place_entity(cfs_rq, se, 0); > se->vruntime -= cfs_rq->min_vruntime; > } > + > + /* > + * Other scheduling classes are not interested in fair's hrtick timer. > + */ > + if (task_current(rq, p) && sched_feat(HRTICK)) > + hrtick_clear(rq); > + > } May be... but in this case, perhaps instead we should teach dequeue_task_fair() or put_prev_task_fair() to do this. Then we can probably remove __schedule()->hrtick_clear(). I simply can't understand the current hrtick logic. For example, why dequeue_task_fair() does hrtick_update() ? OK, probably because "nr_running < sched_nr_latency" can become true. But at least this doesn't make sense to me when p == rq->curr, say, __schedule() path. Hmm. In any case, how it is possible to do hrtick_start() with rq->lock held? hrtimer_restart() may want to wakeup_softirqd(). Peter, Ingo, does this code really work? SCHED_FEAT(HRTICK) == 0 by default, and afaics you can't change it without CONFIG_SCHED_DEBUG. Confused... Oleg.