From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752047AbbKIKgN (ORCPT ); Mon, 9 Nov 2015 05:36:13 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:38093 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750772AbbKIKgM (ORCPT ); Mon, 9 Nov 2015 05:36:12 -0500 Date: Mon, 9 Nov 2015 11:36:06 +0100 From: Peter Zijlstra To: Byungchul Park Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, fweisbec@gmail.com, tglx@linutronix.de Subject: Re: [PATCH v4 2/2] sched: consider missed ticks in full NOHZ Message-ID: <20151109103606.GJ17308@twins.programming.kicks-ass.net> References: <1444816056-11886-1-git-send-email-byungchul.park@lge.com> <1444816056-11886-3-git-send-email-byungchul.park@lge.com> <20151102161016.GS3604@twins.programming.kicks-ass.net> <20151109023654.GA4164@byungchulpark-X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151109023654.GA4164@byungchulpark-X58A-UD3R> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 09, 2015 at 11:36:54AM +0900, Byungchul Park wrote: > On Mon, Nov 02, 2015 at 05:10:16PM +0100, Peter Zijlstra wrote: > > On Wed, Oct 14, 2015 at 06:47:36PM +0900, byungchul.park@lge.com wrote: > > > --- a/kernel/sched/fair.c > > > +++ b/kernel/sched/fair.c > > > @@ -4428,7 +4428,7 @@ static void update_idle_cpu_load(struct rq *this_rq) > > > > So if one were to read the comment above update_idle_cpu_load() one > > would find there's a problem with jiffy based accounting. > > > > > /* > > > * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed. > > > */ > > > -void update_cpu_load_nohz(void) > > > +void update_cpu_load_nohz(int active) > > > > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > > index 7c7ec45..515edf3 100644 > > > --- a/kernel/time/tick-sched.c > > > +++ b/kernel/time/tick-sched.c > > > > > -static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now) > > > +static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now, int active) > > > { > > > /* Update jiffies first */ > > > tick_do_update_jiffies64(now); > > > - update_cpu_load_nohz(); > > > + update_cpu_load_nohz(active); > > > > > > calc_load_exit_idle(); > > > touch_softlockup_watchdog(); > > > > And we could solve all that nicely if we pull up the hrtimer_forward() > > result from tick_nohz_restart(), that way we have the actual number of > > ticks lost on this cpu, and no need to start guessing about it. > > hello, > > are you talking about the lag between writer and reader for jiffies? > i think your proposal can solve the problem of update_cpu_load_nohz(). > but it's still hard to care the cases of update_idle_cpu_load() > and update_cpu_load_active() even by the way you proposed. > > do you think it would be ok even if it solves only one case? > update_idle_cpu_load() still need to guess about it. is there something > i missed? or did i mis-understand what you intend? I was thinking of getting rid of rq->last_load_update_tick entirely. If we can pass in how many (local) ticks were lost on this cpu, we don't have to rely on the jiffy counter at all.