Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754007Ab3JBQWm (ORCPT ); Wed, 2 Oct 2013 12:22:42 -0400 Received: from mail-wg0-f53.google.com ([74.125.82.53]:55136 "EHLO mail-wg0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753256Ab3JBQWc (ORCPT ); Wed, 2 Oct 2013 12:22:32 -0400 Message-ID: <524C4845.5060800@linaro.org> Date: Wed, 02 Oct 2013 18:22:29 +0200 From: Daniel Lezcano User-Agent: Mozilla/5.0 (X11; Linux i686; rv:24.0) Gecko/20100101 Thunderbird/24.0 MIME-Version: 1.0 To: Frederic Weisbecker CC: Thomas Gleixner , Stephen Boyd , LKML , patches@linaro.org, "linaro-kernel@lists.linaro.org" Subject: Re: [PATCH V2] tick: Make sleep length calculation more accurate References: <1380709565-22153-1-git-send-email-daniel.lezcano@linaro.org> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2352 Lines: 64 On 10/02/2013 05:57 PM, Frederic Weisbecker wrote: > 2013/10/2 Daniel Lezcano : >> The sleep_length is computed in the tick_nohz_stop_sched_tick function but it >> is used later in the code with in between the local irq enabled. >> >> cpu_idle_loop >> tick_nohz_idle_enter [ exits with local irq enabled ] >> __tick_nohz_idle_enter >> tick_nohz_stop_sched_tick >> ... >> >> arch_cpu_idle >> menu_select [ uses here 'sleep_length' ] >> ... >> >> Between the computation of the sleep length and its usage, some interrupts >> may occur, making the sleep length shorter than actually it is because of the >> interrupt processing > > So, do you mean that the ts->sleep_length would return a value that is too long > given that the CPU already spent some time to service the irqs since we computed > the sleep length in tick_nohz_idle_enter()? > > But then tick_nohz_irq_exit() should take care of that as it calls > again tick_nohz_stop_sched_tick(). > So I'm a bit confused. > >> or different if the timer itself expired. > > Same here, if the timer expired, it triggers an interrupt which can do > two things: > > 1) reprogram a new timer and this recompute sleep_length > 2) set_need_resched() and then exit the idle loop, so arch_cpu_idle() won't even > be called. Or the timer interrupts hlt, but then menu_select() was > called before. > > So I probably missed something here. No you did not :) Indeed... At the first glance, this issue sounded so obvious I suspected there must be a trick somewhere but I did not think to look at the irq_exit, the code is very complex. Thanks for clarifying this. For my personal information, is there any particular reason to set an intermediate 'sleep_length' in tick_nohz_stop_sched_tick instead of doing what does this patch ? Thanks -- Daniel -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog -- 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/