From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751417AbcBLQQE (ORCPT ); Fri, 12 Feb 2016 11:16:04 -0500 Received: from mail-lb0-f194.google.com ([209.85.217.194]:33901 "EHLO mail-lb0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750825AbcBLQQC (ORCPT ); Fri, 12 Feb 2016 11:16:02 -0500 MIME-Version: 1.0 In-Reply-To: References: <3071836.JbNxX8hU6x@vostro.rjw.lan> <56B93548.9090006@linaro.org> <5387313.xAhVpzgZCg@vostro.rjw.lan> <56BA8C29.4090905@linaro.org> <20160211115959.GI6357@twins.programming.kicks-ass.net> <56BCBF7C.2080404@linaro.org> <20160211173033.GP6357@twins.programming.kicks-ass.net> <56BCD864.6030207@linaro.org> <20160212141009.GT6357@twins.programming.kicks-ass.net> Date: Fri, 12 Feb 2016 17:15:59 +0100 X-Google-Sender-Auth: VTOAnjwgaTHxYD0KjH6094pE6SM Message-ID: Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks From: "Rafael J. Wysocki" To: Peter Zijlstra Cc: Steve Muckle , "Rafael J. Wysocki" , Linux PM list , Linux Kernel Mailing List , Srinivas Pandruvada , Viresh Kumar , Juri Lelli , Thomas Gleixner 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 Fri, Feb 12, 2016 at 5:01 PM, Rafael J. Wysocki wrote: > On Fri, Feb 12, 2016 at 3:10 PM, Peter Zijlstra wrote: >> On Thu, Feb 11, 2016 at 10:52:20AM -0800, Steve Muckle wrote: >>> On 02/11/2016 09:30 AM, Peter Zijlstra wrote: >>> >> My concern above is that pokes are guaranteed to keep occurring when >>> >> > there is only RT or DL activity so nothing breaks. >>> > >>> > The hook in their respective tick handler should ensure stuff is called >>> > sporadically and isn't stalled. >>> >>> But that's only true if the RT/DL tasks happen to be running when the >>> tick arrives right? >>> >>> Couldn't we have RT/DL activity which doesn't overlap with the tick? And >>> if no CFS tasks happen to be executing on that CPU, we'll never trigger >>> the cpufreq update. This could go on for an arbitrarily long time >>> depending on the periodicity of the work. >> >> Possible yes, but why do we care? Such a CPU would be so much idle that >> cpufreq doesn't matter one way or another, right? > > Well, in theory you can get 50% or so of the time active in bursts > that happen to fit between ticks. If we happen to do those in the > lowest P-state, we may burn more energy than necessary on platforms > where more idle is preferred. At least intel_pstate should be able to figure out which P-state to use then on the APERF/MPERF basis.