Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1168604AbdDXKg3 (ORCPT ); Mon, 24 Apr 2017 06:36:29 -0400 Received: from mail.santannapisa.it ([193.205.80.98]:59069 "EHLO mail.santannapisa.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1167652AbdDXKgW (ORCPT ); Mon, 24 Apr 2017 06:36:22 -0400 Date: Mon, 24 Apr 2017 12:36:15 +0200 From: Luca Abeni To: Juri Lelli Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Ingo Molnar , Claudio Scordino , Steven Rostedt , Tommaso Cucinotta , Daniel Bristot de Oliveira , Mathieu Poirier Subject: Re: [PATCH] sched/deadline: fix switching to -deadline Message-ID: <20170424123615.1d2033c9@luca> In-Reply-To: <20170424101624.GA13394@e106622-lin> References: <1492716656-5362-1-git-send-email-luca.abeni@santannapisa.it> <20170421093926.GQ23862@e106622-lin> <20170421114240.0ef43522@nowhere> <20170421094729.GR23862@e106622-lin> <20170421115907.4044665a@nowhere> <20170421102659.GS23862@e106622-lin> <20170421210833.1837de6f@sweethome> <20170424101624.GA13394@e106622-lin> X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3647 Lines: 81 On Mon, 24 Apr 2017 11:16:24 +0100 Juri Lelli wrote: > On 21/04/17 21:08, Luca Abeni wrote: > > On Fri, 21 Apr 2017 11:26:59 +0100 > > Juri Lelli wrote: > > > On 21/04/17 11:59, Luca Abeni wrote: > > > > On Fri, 21 Apr 2017 10:47:29 +0100 > > > > Juri Lelli wrote: > > > > [...] > > > > > > > > *dl_se, update_dl_entity(dl_se, pi_se); > > > > > > > > else if (flags & ENQUEUE_REPLENISH) > > > > > > > > replenish_dl_entity(dl_se, pi_se); > > > > > > > > + else if ((flags & ENQUEUE_RESTORE) && > > > > > > > > > > > > > > Not sure I understand how this works. AFAICT we are doing > > > > > > > __sched_setscheduler() when we want to catch the case of > > > > > > > a new dl_entity (SCHED_{OTHER,FIFO} -> SCHED_DEADLINE}, > > > > > > > but queue_flags (which are passed to enqueue_task()) > > > > > > > don't seem to have ENQUEUE_RESTORE set? > > > > > > > > > > > > I was under the impression sched_setscheduler() sets > > > > > > ENQUEUE_RESTORE... > > > > > > > > > > Oh, I think it works "by coincidence", as ENQUEUE_RESTORE == > > > > > DEQUEUE_SAVE == 0x02 ? :) > > > > > > > > Not sure if this is a conincidence... By looking at the > > > > comments in sched/sched.h I got the impression the two values > > > > match by design (and __sched_setscheduler() is using this > > > > property to simplify the code :) > > > > > > Yep, right. > > > > > > Do you think we might get into trouble with do_set_cpus_allowed()? > > > Can it happen that we change a task affinity while its deadline > > > is in the past? > > > > Well, double thinking about it, this is an interesting problem... > > What do we want to do with do_set_cpus_allowed()? (I mean: what is > > the expected behaviour?) > > > > With this patch, if a task is moved to a different runqueue when its > > deadline is in the past (because we are doing gEDF, or because of > > timer granularity issues) its scheduling deadline is reinitialized > > to current time + relative deadline... I think this makes perfect > > sense, doesn't it? > > > > Mmm, I don't think we will (with this patch) actually reinitialize the > deadline when a "normal" gEDF migration happen (push/pull), as > (de)activate_task() have no flag set. Which brings the question, > should we actually take care of this corner case (as what you say > makes sense to me too)? I might be misunderstanding the problem, here... Are you talking about do_set_cpus_allowed()? Or about push/pull migrations happening because of the gEDF algorithm? If you are referring to do_set_cpus_allowed, this is my understanding: 1) If do_set_cpus_allowed() is called on a queued task, then dequeue_task() with DEQUEUE_SAVE is called, followed by enqueue_task() with ENQUEUE_RESTORE... So, if the deadline is in the past it is correctly reinitialized 2) If do_set_cpus_allowed() is called on a non-queued task, this means the task is blocked, no? So, when it will wake up enqueue_dl_entity() will invoke update_dl_entity() that will check if the deadline is in the past. If you are referring to push/pull migrations due to gEDF, then enqueue_dl_entity() will be invoked with "flags" = 0, so the deadline will not be changed (and this is correct: we do not want to initialize / change tasks' deadlines during gEDF migrations). In my previous email, with "a task is moved to a different runqueue" I wanted to say that the taks is forced to moved to a different runqueue because its affinity is changed; I did not want to talk about "regular migrations" due to the push/pull (gEDF) mechanism. Luca