From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754858AbcEXNwX (ORCPT ); Tue, 24 May 2016 09:52:23 -0400 Received: from mail-lf0-f45.google.com ([209.85.215.45]:35629 "EHLO mail-lf0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752419AbcEXNwW (ORCPT ); Tue, 24 May 2016 09:52:22 -0400 MIME-Version: 1.0 In-Reply-To: <20160524133642.GH27946@e105550-lin.cambridge.arm.com> References: <1464001138-25063-1-git-send-email-morten.rasmussen@arm.com> <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> From: Vincent Guittot Date: Tue, 24 May 2016 15:52:00 +0200 Message-ID: Subject: Re: [PATCH 06/16] sched: Disable WAKE_AFFINE for asymmetric configurations To: Morten Rasmussen Cc: Peter Zijlstra , "mingo@redhat.com" , Dietmar Eggemann , Yuyang Du , mgalbraith@suse.de, linux-kernel Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.