From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754172AbcEYJLX (ORCPT ); Wed, 25 May 2016 05:11:23 -0400 Received: from foss.arm.com ([217.140.101.70]:33382 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751278AbcEYJLU (ORCPT ); Wed, 25 May 2016 05:11:20 -0400 Date: Wed, 25 May 2016 10:12:01 +0100 From: Morten Rasmussen To: Vincent Guittot Cc: Peter Zijlstra , "mingo@redhat.com" , Dietmar Eggemann , Yuyang Du , mgalbraith@suse.de, linux-kernel Subject: Re: [PATCH 06/16] sched: Disable WAKE_AFFINE for asymmetric configurations Message-ID: <20160525091200.GJ27946@e105550-lin.cambridge.arm.com> References: <1464001138-25063-7-git-send-email-morten.rasmussen@arm.com> <20160524102928.GF27946@e105550-lin.cambridge.arm.com> <20160524131610.GG27946@e105550-lin.cambridge.arm.com> <20160524133642.GH27946@e105550-lin.cambridge.arm.com> <20160524150204.GI27946@e105550-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 24, 2016 at 05:53:27PM +0200, Vincent Guittot wrote: > On 24 May 2016 at 17:02, Morten Rasmussen wrote: > > On Tue, May 24, 2016 at 03:52:00PM +0200, Vincent Guittot wrote: > >> On 24 May 2016 at 15:36, Morten Rasmussen wrote: > >> > On Tue, May 24, 2016 at 03:27:05PM +0200, Vincent Guittot wrote: > >> >> On 24 May 2016 at 15:16, Morten Rasmussen wrote: > >> >> > On Tue, May 24, 2016 at 02:12:38PM +0200, Vincent Guittot wrote: > >> >> >> On 24 May 2016 at 12:29, Morten Rasmussen wrote: > >> >> >> > On Tue, May 24, 2016 at 11:10:28AM +0200, Vincent Guittot wrote: > >> >> >> >> On 23 May 2016 at 12:58, Morten Rasmussen wrote: > >> >> >> >> > If the system has cpu of different compute capacities (e.g. big.LITTLE) > >> >> >> >> > let affine wakeups be constrained to cpus of the same type. > >> >> >> >> > >> >> >> >> Can you explain why you don't want wake affine with cpus with > >> >> >> >> different compute capacity ? > >> >> >> > > >> >> >> > I should have made the overall idea a bit more clear. The idea is to > >> >> >> > deal with cross-capacity migrations in the find_idlest_{group, cpu}{} > >> >> >> > path so we don't have to touch select_idle_sibling(). > >> >> >> > select_idle_sibling() is critical for wake-up latency, and I'm assumed > >> >> >> > that people wouldn't like adding extra overhead in there to deal with > >> >> >> > capacity and utilization. > >> >> >> > >> >> >> So this means that we will never use the quick path of > >> >> >> select_idle_sibling for cross capacity migration but always the one > >> >> >> with extra overhead? > >> >> > > >> >> > Yes. select_idle_sibling() is only used to choose among equal capacity > >> >> > cpus (capacity_orig). > >> >> > > >> >> >> Patch 9 adds more tests for enabling wake_affine path. Can't it also > >> >> >> be used for cross capacity migration ? so we can use wake_affine if > >> >> >> the task or the cpus (even with different capacity) doesn't need this > >> >> >> extra overhead > >> >> > > >> >> > The test in patch 9 is to determine whether we are happy with the > >> >> > capacity of the previous cpu, or we should go look for one with more > >> >> > capacity. I don't see how we can use select_idle_sibling() unmodified > >> >> > for sched domains containing cpus of different capacity to select an > >> >> > appropriate cpu. It is just picking an idle cpu, it might have high > >> >> > capacity or low, it wouldn't care. > >> >> > > >> >> > How would you avoid the overhead of checking capacity and utilization of > >> >> > the cpus and still pick an appropriate cpu? > >> >> > >> >> My point is that there is some wake up case where we don't care about > >> >> the capacity and utilization of cpus even for cross capacity migration > >> >> and we will never take benefit of this fast path. > >> >> You have added an extra check for setting want_affine in patch 9 which > >> >> uses capacity and utilization of cpu to disable this fast path when a > >> >> task needs more capacity than available. Can't you use this function > >> >> to disable the want_affine for cross-capacity migration situation that > >> >> cares of the capacity and need the full scan of sched_domain but keep > >> >> it enable for other cases ? > >> > > >> > It is not clear to me what the other cases are. What kind of cases do > >> > you have in mind? > >> > >> As an example, you have a task A that have to be on a big CPU because > >> of the requirement of compute capacity, that wakes up a task B that > >> can run on any cpu according to its utilization. The fast wake up path > >> is fine for task B whatever prev cpu is. > > > > In that case, we will take always take fast path (select_idle_sibling()) > > for task B if wake_wide() allows it, which should be fine. > > Even if want_affine is set, the wake up of task B will not use the fast path. > The affine_sd will not be set because the sched_domain, which have > both cpus, will not have the SD_WAKE_AFFINE flag according to this > patch, isn't it ? > So task B can't use the fast path whereas nothing prevent him to take > benefit of it > > Am I missing something ? No, I think you are right. Very good point. The cpumask test with sched_domain_span() will of cause return false. So yes, in this case the slow path is taken. It isn't wrong as such, just slower for asymmetric capacity systems :-) It is clearly not as optimized for asymmetric capacity systems as it could be, but my focus was to not ruin existing behaviour and minimize overhead for others. There are a lot of different routes through those conditions in the first half of select_task_rq_fair() that aren't obvious. I worry that some users depend on them and that I don't see/understand all of them. If people agree on changing things, it is fine with me. I just tried to avoid getting the patches shot down on that account ;-)