From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752311Ab3LJTHr (ORCPT ); Tue, 10 Dec 2013 14:07:47 -0500 Received: from mail-we0-f177.google.com ([74.125.82.177]:37586 "EHLO mail-we0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751237Ab3LJTHq (ORCPT ); Tue, 10 Dec 2013 14:07:46 -0500 Message-ID: <52A76680.5000808@linaro.org> Date: Tue, 10 Dec 2013 20:07:44 +0100 From: Daniel Lezcano User-Agent: Mozilla/5.0 (X11; Linux i686; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 To: Alex Shi , Linux Kernel Mailing List , Mike Galbraith Subject: Re: [question] sched: idle_avg and migration latency References: <52A6FB5C.7010706@linaro.org> <52A73121.30309@linaro.org> In-Reply-To: <52A73121.30309@linaro.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/10/2013 04:20 PM, Alex Shi wrote: > CC to MikeG, he written this part. :) > I try to explain sth I know. I am sorry if my understanding incorrect. > > On 12/10/2013 07:30 PM, Daniel Lezcano wrote: >> >> Hi All, >> >> I am trying to understand how is computed the idle_avg and how it is >> used regarding the migration latency. >> >> 1. What is the sysctl_sched_migration_cost value ? It is initialized to >> 500000UL. Is it an arbitrarily chosen value ? Could it change depending >> on the hardware performances ? > > current sysctl_sched_mirgration_cost is 0.5ms, used to limit > overscheduling. Guess it is a kind of arbitrary. But it can be rewrite > at /proc/sys/kernel/sched_migration_cost_ns. > So if you find some new suitable value in particular scenario. guess > PeterZ like to modify it. :) > >> >> >> 2. The idle_balance function checks: >> >> if (this_rq->avg_idle < sysctl_sched_migration_cost) >> return 0; >> >> IIUC, it is not worth to migrate a task to this cpu as we expect to run >> another task before we can pull a task to the current cpu, right ? > > No, that used to prevent every idle_balance cause a task migration if > idle balance happens too much and too quick, -- frequency more than task > migration limitation. >> >> Then if there is no task to balance we will enter idle, thus we >> initialize the idle_stamp to the current clock. > > If we pulled task, we will restart frequency calculation by set > idle_stamp = 0; > or if new task adding this rq, allow more idle_balance. Thanks Alex for the explanation. >> When another task is woken up with the ttwu_do_wakeup, the duration of >> the idle time is computed in there: >> >> if (rq->idle_stamp) { >> u64 delta = rq_clock(rq) - rq->idle_stamp; >> u64 max = 2*sysctl_sched_migration_cost; >> >> if (delta > max) >> rq->avg_idle = max; >> else >> update_avg(&rq->avg_idle, delta); >> rq->idle_stamp = 0; >> } >> >> Why is the 'delta' leveraged by 'max' ? >> >> >> 3. And finally the function update_avg does: >> >> s64 diff = sample - *avg; >> *avg += diff >> 3; >> >> Why is diff >> 3 used instead of the number of values ? > > It is a kind of decay. but has no idea of why this value '3'. Guess > MikeG has some reason. >> >> Thanks in advance for any answers >> >> -- Daniel >> > > -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog