Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754892AbcC3Tfz (ORCPT ); Wed, 30 Mar 2016 15:35:55 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:33838 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753181AbcC3Tfy (ORCPT ); Wed, 30 Mar 2016 15:35:54 -0400 Date: Wed, 30 Mar 2016 21:35:44 +0200 From: Peter Zijlstra To: Steve Muckle Cc: Dietmar Eggemann , Ingo Molnar , linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, "Rafael J. Wysocki" , Vincent Guittot , Morten Rasmussen , Juri Lelli , Patrick Bellasi , Michael Turquette Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg() Message-ID: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56F98832.3030207@linaro.org> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 710 Lines: 13 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.