Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751584Ab3JEJxR (ORCPT ); Sat, 5 Oct 2013 05:53:17 -0400 Received: from mail-we0-f177.google.com ([74.125.82.177]:48617 "EHLO mail-we0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751408Ab3JEJxP (ORCPT ); Sat, 5 Oct 2013 05:53:15 -0400 Date: Sat, 5 Oct 2013 11:53:12 +0200 From: Frederic Weisbecker To: Daniel Lezcano 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 Message-ID: <20131005095310.GB28647@localhost.localdomain> References: <1380709565-22153-1-git-send-email-daniel.lezcano@linaro.org> <524C4845.5060800@linaro.org> <20131002164221.GI7941@localhost.localdomain> <524C5FFB.6020504@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <524C5FFB.6020504@linaro.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2983 Lines: 77 On Wed, Oct 02, 2013 at 08:03:39PM +0200, Daniel Lezcano wrote: > On 10/02/2013 06:42 PM, Frederic Weisbecker wrote: > >On Wed, Oct 02, 2013 at 06:22:29PM +0200, Daniel Lezcano wrote: > >>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 ? > > > >May be we could do it that way yeah. Is menu_select() called only there? > >I don't know how much difference that would make. > > Yes, it is called just one time in all the code. The benefit would > be just to cleanup a field in the struct tick_sched. Yeah, why not. Thanks. > > > -- > 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/