Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756511AbaDKJyP (ORCPT ); Fri, 11 Apr 2014 05:54:15 -0400 Received: from mail-oa0-f43.google.com ([209.85.219.43]:64533 "EHLO mail-oa0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754232AbaDKJyM (ORCPT ); Fri, 11 Apr 2014 05:54:12 -0400 MIME-Version: 1.0 In-Reply-To: <20140410145621.GC27654@localhost.localdomain> References: <20140410145621.GC27654@localhost.localdomain> Date: Fri, 11 Apr 2014 15:24:11 +0530 Message-ID: Subject: Re: [Query]: tick-sched: can idle_active be false in tick_nohz_idle_exit()? From: Viresh Kumar To: Frederic Weisbecker Cc: Thomas Gleixner , Linux Kernel Mailing List , Lists linaro-kernel Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10 April 2014 20:26, Frederic Weisbecker wrote: > When a dynticks idle CPU is woken up (typically with an IPI), tick_nohz_stop_idle() > is called on interrupt entry but, because this is a waking up IPI, tick_nohz_start_idle() > won't be called. The reason is that need_resched() prevents tick_nohz_irq_exit() to be > called in irq_exit(). > > After all if we know that the CPU is going to exit the idle task, we don't need to account > any more idle time. We also don't need to retry to enter in dynticks idle mode since we > are going to restart the tick with tick_nohz_idle_exit(). > > So in case of wake up IPIs, we may end up with !ts->idle_active in tick_nohz_idle_exit() :) > > I must confess this is not obvious. I agree.. I didn't had a clue that this can happen. Good that I asked this question :) > It confused me as well when I met that part. A small > comment in tick_nohz_idle_exit() would be welcome ;) Looks hard. I tried for a small comment and this is the smallest I could get: diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index c2d868d..26cf5bb 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -925,6 +925,22 @@ void tick_nohz_idle_exit(void) ts->inidle = 0; + /* + * Can idle_active be false here? + * Ideally this would be the sequence of calls: + * - tick_nohz_idle_enter(), i.e. idle_active = true; + * - local_irq_disable() + * - IDLE + * - wake up due to IPI or other interrupt + * - local_irq_enable() + * - tick_nohz_irq_enter(), i.e. idle_active = false; + * - tick_nohz_irq_exit(), i.e. idle_active = true; This is not called + * in case of IPI's as need_resched() will prevent that in + * tick_irq_exit(), as we don't need to account any more for idle time + * or try to enter dyntics mode (We are going to exit idle state). + * + * - tick_nohz_idle_exit() + */ if (ts->idle_active || ts->tick_stopped) now = ktime_get(); I am preparing a cleanup patchset (separate from the timer/hrtimers 36 patch set) for kernel/time/ and will add this change to that.. I am waiting for the merge window to close and Thomas to comment on my timers/hrtimers patches first. Only then will I send another 40 :) -- viresh -- 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/