From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752100AbcF3Mwb (ORCPT ); Thu, 30 Jun 2016 08:52:31 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:33228 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751424AbcF3Mwa (ORCPT ); Thu, 30 Jun 2016 08:52:30 -0400 Date: Thu, 30 Jun 2016 14:52:26 +0200 From: Frederic Weisbecker To: Peter Zijlstra Cc: LKML , Ingo Molnar , Mike Galbraith , Thomas Gleixner Subject: Re: [PATCH 2/3] sched: Unloop sched avg decaying Message-ID: <20160630125225.GA32568@lerouge> References: <1465918082-27005-1-git-send-email-fweisbec@gmail.com> <1465918082-27005-3-git-send-email-fweisbec@gmail.com> <20160614155842.GJ30921@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160614155842.GJ30921@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 14, 2016 at 05:58:42PM +0200, Peter Zijlstra wrote: > On Tue, Jun 14, 2016 at 05:28:01PM +0200, Frederic Weisbecker wrote: > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 385c947..0c0578a 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -666,17 +666,17 @@ bool sched_can_stop_tick(struct rq *rq) > > void sched_avg_update(struct rq *rq) > > { > > s64 period = sched_avg_period(); > > + s64 delta; > > + u64 rem; > > + int pending; > > > > + delta = (s64)(rq_clock(rq) - rq->age_stamp); > > + if (delta <= period) > > + return; > > + > > + pending = div64_u64_rem(delta, period, &rem); > > + rq->age_stamp += delta - rem; > > + rq->rt_avg >>= pending; > > } > > Blergh, and now do the profile on machine that doesn't have major > transistor count dedicated to divisions. Indeed I was afraid of something like that. That said I'm not sure which is worse between iteration or division out of native asm. Would it be worth testing? > > Why not add the division to the nohz exit path only? It would be worse I think because we may exit much more often from nohz than we reach a sched_avg_period(). So the only safe optimization I can do for now is: diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 8b489fc..4632d31 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -626,8 +626,9 @@ bool sched_can_stop_tick(struct rq *rq) void sched_avg_update(struct rq *rq) { s64 period = sched_avg_period(); + int i; - while ((s64)(rq_clock(rq) - rq->age_stamp) > period) { + for (i = 0; (s64)(rq_clock(rq) - rq->age_stamp) > period); i++) { /* * Inline assembly required to prevent the compiler * optimising this loop into a divmod call. @@ -635,8 +636,8 @@ void sched_avg_update(struct rq *rq) */ asm("" : "+rm" (rq->age_stamp)); rq->age_stamp += period; - rq->rt_avg /= 2; } + rq->rt_avg >>= i; } #endif /* CONFIG_SMP */