Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759943AbaJ1Gcm (ORCPT ); Tue, 28 Oct 2014 02:32:42 -0400 Received: from mail-ob0-f169.google.com ([209.85.214.169]:39098 "EHLO mail-ob0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755493AbaJ1DvZ (ORCPT ); Mon, 27 Oct 2014 23:51:25 -0400 MIME-Version: 1.0 In-Reply-To: <1414054881-17713-1-git-send-email-daniel.lezcano@linaro.org> References: <1414054881-17713-1-git-send-email-daniel.lezcano@linaro.org> Date: Tue, 28 Oct 2014 09:21:25 +0530 Message-ID: Subject: Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before idle From: Preeti Murthy To: Daniel Lezcano Cc: "Rafael J. Wysocki" , Nicolas Pitre , "linux-pm@vger.kernel.org" , LKML , Peter Zijlstra , Lists linaro-kernel , patches@linaro.org, Preeti U Murthy Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Daniel, On Thu, Oct 23, 2014 at 2:31 PM, Daniel Lezcano wrote: > When the pmqos latency requirement is set to zero that means "poll in all the > cases". > > That is correctly implemented on x86 but not on the other archs. > > As how is written the code, if the latency request is zero, the governor will > return zero, so corresponding, for x86, to the poll function, but for the > others arch the default idle function. For example, on ARM this is wait-for- > interrupt with a latency of '1', so violating the constraint. This is not true actually. On PowerPC the idle state 0 has an exit_latency of 0. > > In order to fix that, do the latency requirement check *before* calling the > cpuidle framework in order to jump to the poll function without entering > cpuidle. That has several benefits: Doing so actually hurts on PowerPC. Because the idle loop defined for idle state 0 is different from what cpu_relax() does in cpu_idle_loop(). The spinning is more power efficient in the former case. Moreover we also set certain register values which indicate an idle cpu. The ppc_runlatch bits do precisely this. These register values are being read by some user space tools. So we will end up breaking them with this patch My suggestion is very well keep the latency requirement check in kernel/sched/idle.c like your doing in this patch. But before jumping to cpu_idle_loop verify if the idle state 0 has an exit_latency > 0 in addition to your check on the latency_req == 0. If not, you can fall through to the regular path of calling into the cpuidle driver. The scheduler can query the cpuidle_driver structure anyway. What do you think? Regards Preeti U Murthy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/