From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753474AbaAWPXx (ORCPT ); Thu, 23 Jan 2014 10:23:53 -0500 Received: from merlin.infradead.org ([205.233.59.134]:37262 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751001AbaAWPXw (ORCPT ); Thu, 23 Jan 2014 10:23:52 -0500 Date: Thu, 23 Jan 2014 16:23:30 +0100 From: Peter Zijlstra To: Daniel Lezcano Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, pjt@google.com, bsegall@google.com, Jason Low Subject: Re: [PATCH 3/9] sched: Move idle_stamp up to the core Message-ID: <20140123152330.GW30183@twins.programming.kicks-ass.net> References: <20140121111754.580142558@infradead.org> <20140121112258.410353740@infradead.org> <20140123125822.GX31570@twins.programming.kicks-ass.net> <52E129A8.5050503@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52E129A8.5050503@linaro.org> 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 Thu, Jan 23, 2014 at 03:39:36PM +0100, Daniel Lezcano wrote: > On 01/23/2014 01:58 PM, Peter Zijlstra wrote: > >On Tue, Jan 21, 2014 at 12:17:57PM +0100, Peter Zijlstra wrote: > >>From: Daniel Lezcano > >> > >>The idle_balance modifies the idle_stamp field of the rq, making this > >>information to be shared across core.c and fair.c. As we can know if the > >>cpu is going to idle or not with the previous patch, let's encapsulate the > >>idle_stamp information in core.c by moving it up to the caller. The > >>idle_balance function returns true in case a balancing occured and the cpu > >>won't be idle, false if no balance happened and the cpu is going idle. > >> > >>Cc: alex.shi@linaro.org > >>Cc: peterz@infradead.org > >>Cc: mingo@kernel.org > >>Signed-off-by: Daniel Lezcano > >>Signed-off-by: Peter Zijlstra > >>Link: http://lkml.kernel.org/r/1389949444-14821-3-git-send-email-daniel.lezcano@linaro.org > >>--- > >> kernel/sched/core.c | 2 +- > >> kernel/sched/fair.c | 14 ++++++-------- > >> kernel/sched/sched.h | 2 +- > >> 3 files changed, 8 insertions(+), 10 deletions(-) > >> > >>--- a/kernel/sched/core.c > >>+++ b/kernel/sched/core.c > >>@@ -2680,7 +2680,7 @@ static void __sched __schedule(void) > >> pre_schedule(rq, prev); > >> > >> if (unlikely(!rq->nr_running)) > >>- idle_balance(rq); > >>+ rq->idle_stamp = idle_balance(rq) ? 0 : rq_clock(rq); > > > >OK, spotted a problem here.. > > > >So previously idle_stamp was set _before_ actually doing idle_balance(), > >and that was RIGHT, because that way we include the cost of actually > >doing idle_balance() into the idle time. > > > >By not including the cost of idle_balance() you underestimate the 'idle' > >time in that if idle_balance() filled the entire idle time we account 0 > >idle, even though we had 'plenty' of time to run the entire thing. > > > >This leads to not running idle_balance() even though we have the time > >for it. > > Good catch. How did you notice that ? Staring at that code for too long :-) > >So we very much want something like: > > > > > > if (!rq->nr_running) > > rq->idle_stamp = rq_clock(rq); > > > > p = pick_next_task(rq, prev); > > > > if (!is_idle_task(p)) > > rq->idle_stamp = 0; > > Is this code assuming idle_balance() is in pick_next_task ? Yeah, I'm trying to make that work.