Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757273Ab3FSRAs (ORCPT ); Wed, 19 Jun 2013 13:00:48 -0400 Received: from service87.mimecast.com ([91.220.42.44]:33489 "EHLO service87.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757021Ab3FSRAr convert rfc822-to-8bit (ORCPT ); Wed, 19 Jun 2013 13:00:47 -0400 Date: Wed, 19 Jun 2013 18:00:43 +0100 From: Morten Rasmussen To: Arjan van de Ven Cc: David Lang , Ingo Molnar , "alex.shi@intel.com" , "peterz@infradead.org" , "preeti@linux.vnet.ibm.com" , "vincent.guittot@linaro.org" , "efault@gmx.de" , "pjt@google.com" , "linux-kernel@vger.kernel.org" , "linaro-kernel@lists.linaro.org" , "len.brown@intel.com" , "corbet@lwn.net" , Andrew Morton , Linus Torvalds , "tglx@linutronix.de" , Catalin Marinas Subject: Re: power-efficient scheduling design Message-ID: <20130619170042.GH5460@e103034-lin> References: <20130530134718.GB32728@e103034-lin> <20130531105204.GE30394@gmail.com> <20130614160522.GG32728@e103034-lin> <51C07ABC.2080704@linux.intel.com> <51C1D0BB.3040705@linux.intel.com> MIME-Version: 1.0 In-Reply-To: <51C1D0BB.3040705@linux.intel.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-OriginalArrivalTime: 19 Jun 2013 17:00:42.0479 (UTC) FILETIME=[88586FF0:01CE6D0E] X-MC-Unique: 113061918004301801 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2666 Lines: 50 On Wed, Jun 19, 2013 at 04:39:39PM +0100, Arjan van de Ven wrote: > On 6/18/2013 10:47 AM, David Lang wrote: > > > > > It's bad enough trying to guess the needs of the processes, but if you also are reduced to guessing the capabilities of the cores, how can anything be made to work? > > btw one way to look at this is to assume that (with some minimal hinting) > the CPU driver will do the right thing and get you just about the best performance you can get > (that is appropriate for the task at hand)... > ... and don't do anything in the scheduler proactively. If I understand correctly, you mean if your hardware/firmware is fully in control of the p-state selection and changes it fast enough to match the current load, the scheduler doesn't have to care? By fast enough I mean, faster than the scheduler would notice if a cpu was temporarily overloaded at a low p-state. In that case, you wouldn't need cpufreq/p-state hints, and the scheduler would only move tasks between cpus when cpus are fully loaded at their max p-state. > > Now for big.little and other temporary or permanent asymmetries, we may want to > have a "max performance level" type indicator, and that's fair enough > (and this can be dynamic, since it for thermal reasons this can change over time, > but on a somewhat slower timescale) > > > the hints I have in mind are not all that complex; we have the biggest issues today > around task migration (the task migrates to a cold cpu... so a simple notifier chain > on the new cpu as it is accepting a task and we can bump it up), real time tasks > (again, simple notifier chain to get you to a predictably high performance level) > and we're a long way better than we are today in terms of actual problems. > > For all the talk of ondemand (as ARM still uses that today)... that guy puts you in > either the lowest or highest frequency over 95% of the time. Other non-cpufreq solutions > like on Intel are bit more advanced (and will grow more so over time), but even there, > in the grand scheme of things, the scheduler shouldn't have to care anymore with those > two notifiers in place. You would need more than a few hints to implement more advanced capacity management like proposed for the power scheduler. I believe that Intel would benefit as well from guiding the scheduler to idle the right cpu to enable deeper idle states and/or enable turbo-boost for other cpus. 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/