From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755824AbbI3Kpc (ORCPT ); Wed, 30 Sep 2015 06:45:32 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:41266 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753505AbbI3KpX (ORCPT ); Wed, 30 Sep 2015 06:45:23 -0400 Date: Wed, 30 Sep 2015 12:43:43 +0200 From: Peter Zijlstra To: Frederic Weisbecker Cc: byungchul.park@lge.com, mingo@kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de Subject: Re: [RESEND PATCH] sched: consider missed ticks when updating global cpu load Message-ID: <20150930104343.GE2881@worktop.programming.kicks-ass.net> References: <1443171157-23384-1-git-send-email-byungchul.park@lge.com> <20150926131444.GA5507@lerouge> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150926131444.GA5507@lerouge> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Sep 26, 2015 at 03:14:45PM +0200, Frederic Weisbecker wrote: > > when the next tick occurs, update_process_times() -> scheduler_tick() > > -> update_cpu_load_active() is performed, assuming the distance between > > last tick and current tick is 1 tick! it's wrong in this case. thus, > > this abnormal case should be considered in update_cpu_load_active(). > > > > Signed-off-by: Byungchul Park > > --- > > kernel/sched/fair.c | 7 +++++-- > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 4d5f97b..829282f 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -4356,12 +4356,15 @@ void update_cpu_load_nohz(void) > > */ > > void update_cpu_load_active(struct rq *this_rq) > > { > > + unsigned long curr_jiffies = READ_ONCE(jiffies); > > + unsigned long pending_updates; > > unsigned long load = weighted_cpuload(cpu_of(this_rq)); > > /* > > * See the mess around update_idle_cpu_load() / update_cpu_load_nohz(). > > */ > > - this_rq->last_load_update_tick = jiffies; > > - __update_cpu_load(this_rq, load, 1); > > + pending_updates = curr_jiffies - this_rq->last_load_update_tick; > > + this_rq->last_load_update_tick = curr_jiffies; > > + __update_cpu_load(this_rq, load, pending_updates); > > } > > That's right but __update_cpu_load() doesn't handle correctly pending updates > with non-zero loads. Currently, pending updates are wheeled through decay_load_missed() > that assume it's all about idle load. > > But in the cases you've enumerated, as well as in the nohz full case, missed pending > updates can be about buzy loads. > > I think we need to fix update_cpu_load() to handle that first, or your fix is > going to make things worse. Its worse than that, the whole call chain of update_process_times() fully assumes a single tick, fixing just the one function deep down to handle more than 1 tick is ass backwards.