Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752433AbaFDLXi (ORCPT ); Wed, 4 Jun 2014 07:23:38 -0400 Received: from fw-tnat.austin.arm.com ([217.140.110.23]:22804 "EHLO collaborate-mta1.arm.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752216AbaFDLXh (ORCPT ); Wed, 4 Jun 2014 07:23:37 -0400 Date: Wed, 4 Jun 2014 12:23:42 +0100 From: Morten Rasmussen To: Vincent Guittot Cc: Peter Zijlstra , "mingo@kernel.org" , "linux-kernel@vger.kernel.org" , "linux@arm.linux.org.uk" , "linux-arm-kernel@lists.infradead.org" , "preeti@linux.vnet.ibm.com" , "efault@gmx.de" , "nicolas.pitre@linaro.org" , "linaro-kernel@lists.linaro.org" , "daniel.lezcano@linaro.org" Subject: Re: [PATCH v2 08/11] sched: get CPU's activity statistic Message-ID: <20140604112342.GN29593@e103034-lin> References: <20140528121001.GI19967@e103034-lin> <20140528154703.GJ19967@e103034-lin> <20140603155007.GZ30445@twins.programming.kicks-ass.net> <20140604080809.GK30445@twins.programming.kicks-ass.net> <20140604101724.GD11096@twins.programming.kicks-ass.net> <20140604103619.GL29593@e103034-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 04, 2014 at 12:07:29PM +0100, Vincent Guittot wrote: > On 4 June 2014 12:36, Morten Rasmussen wrote: > > On Wed, Jun 04, 2014 at 11:17:24AM +0100, Peter Zijlstra wrote: > >> On Wed, Jun 04, 2014 at 11:32:10AM +0200, Vincent Guittot wrote: > >> > On 4 June 2014 10:08, Peter Zijlstra wrote: > >> > > On Wed, Jun 04, 2014 at 09:47:26AM +0200, Vincent Guittot wrote: > >> > >> On 3 June 2014 17:50, Peter Zijlstra wrote: > >> > >> > On Wed, May 28, 2014 at 04:47:03PM +0100, Morten Rasmussen wrote: > >> > >> >> Since we may do periodic load-balance every 10 ms or so, we will perform > >> > >> >> a number of load-balances where runnable_avg_sum will mostly be > >> > >> >> reflecting the state of the world before a change (new task queued or > >> > >> >> moved a task to a different cpu). If you had have two tasks continuously > >> > >> >> on one cpu and your other cpu is idle, and you move one of the tasks to > >> > >> >> the other cpu, runnable_avg_sum will remain unchanged, 47742, on the > >> > >> >> first cpu while it starts from 0 on the other one. 10 ms later it will > >> > >> >> have increased a bit, 32 ms later it will be 47742/2, and 345 ms later > >> > >> >> it reaches 47742. In the mean time the cpu doesn't appear fully utilized > >> > >> >> and we might decide to put more tasks on it because we don't know if > >> > >> >> runnable_avg_sum represents a partially utilized cpu (for example a 50% > >> > >> >> task) or if it will continue to rise and eventually get to 47742. > >> > >> > > >> > >> > Ah, no, since we track per task, and update the per-cpu ones when we > >> > >> > migrate tasks, the per-cpu values should be instantly updated. > >> > >> > > >> > >> > If we were to increase per task storage, we might as well also track > >> > >> > running_avg not only runnable_avg. > >> > >> > >> > >> I agree that the removed running_avg should give more useful > >> > >> information about the the load of a CPU. > >> > >> > >> > >> The main issue with running_avg is that it's disturbed by other tasks > >> > >> (as point out previously). As a typical example, if we have 2 tasks > >> > >> with a load of 25% on 1 CPU, the unweighted runnable_load_avg will be > >> > >> in the range of [100% - 50%] depending of the parallelism of the > >> > >> runtime of the tasks whereas the reality is 50% and the use of > >> > >> running_avg will return this value > >> > > > >> > > I'm not sure I see how 100% is possible, but yes I agree that runnable > >> > > can indeed be inflated due to this queueing effect. > >> > >> Let me explain the 75%, take any one of the above scenarios. Lets call > >> the two tasks A and B, and let for a moment assume A always wins and > >> runs first, and then B. > >> > >> So A will be runnable for 25%, B otoh will be runnable the entire time A > >> is actually running plus its own running time, giving 50%. Together that > >> makes 75%. > >> > >> If you release the assumption that A runs first, but instead assume they > >> equally win the first execution, you get them averaging at 37.5% each, > >> which combined will still give 75%. > > > > But that is assuming that the first task gets to run to completion of it > > busy period. If it uses up its sched_slice and we switch to the other > > tasks, they both get to wait. > > > > For example, if the sched_slice is 5 ms and the busy period is 10 ms, > > the execution pattern would be: A, B, A, B, idle, ... In that case A is > > runnable for 15 ms and B is for 20 ms. Assuming that the overall period > > is 40 ms, the A runnable is 37.5% and B is 50%. > > The exact value for your scheduling example above is: > A runnable will be 47% and B runnable will be 60% (unless i make a > mistake in my computation) I get: A: 15/40 ms = 37.5% B: 20/40 ms = 50% Schedule: | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | A: run rq run ----------- sleeping ------------- run B: rq run rq run ---- sleeping ------------- rq > and CPU runnable will be 60% too rq->avg.runnable_avg_sum should be 50%. You have two tasks running for 20 ms every 40 ms. Right? Morten -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/