Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932111AbcCaJ1q (ORCPT ); Thu, 31 Mar 2016 05:27:46 -0400 Received: from mail-lf0-f47.google.com ([209.85.215.47]:34358 "EHLO mail-lf0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755491AbcCaJ1o (ORCPT ); Thu, 31 Mar 2016 05:27:44 -0400 MIME-Version: 1.0 In-Reply-To: <20160330193544.GD407@worktop> References: <1458606068-7476-1-git-send-email-smuckle@linaro.org> <56F91D56.4020007@arm.com> <56F95D10.4070400@linaro.org> <56F97856.4040804@arm.com> <56F98832.3030207@linaro.org> <20160330193544.GD407@worktop> From: Vincent Guittot Date: Thu, 31 Mar 2016 11:27:22 +0200 Message-ID: Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg() To: Peter Zijlstra Cc: Steve Muckle , Dietmar Eggemann , Ingo Molnar , linux-kernel , "linux-pm@vger.kernel.org" , "Rafael J. Wysocki" , Morten Rasmussen , Juri Lelli , Patrick Bellasi , Michael Turquette Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 854 Lines: 16 On 30 March 2016 at 21:35, Peter Zijlstra wrote: > On Mon, Mar 28, 2016 at 12:38:26PM -0700, Steve Muckle wrote: >> Without covering all the paths where CFS utilization changes it's >> possible to have to wait up to a tick to act on some changes, since the >> tick is the only guaranteed regularly-occurring instance of the hook. >> That's an unacceptable amount of latency IMO... > > Note that even with your patches that might still be the case. Remote > wakeups might not happen on the destination CPU at all, so it might not > be until the next tick (which always happens locally) that we'll > 'observe' the utilization change brought with the wakeups. > > We could force all the remote wakeups to IPI the destination CPU, but > that comes at a significant performance cost. Isn't a reschedule ipi already sent in this case ?