From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932923AbcBIRaK (ORCPT ); Tue, 9 Feb 2016 12:30:10 -0500 Received: from mail-pf0-f175.google.com ([209.85.192.175]:36314 "EHLO mail-pf0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755737AbcBIRaI (ORCPT ); Tue, 9 Feb 2016 12:30:08 -0500 Subject: Re: [PATCH v3 0/6] CPUs capacity information for heterogeneous systems To: Juri Lelli References: <1454500799-18451-1-git-send-email-juri.lelli@arm.com> <56B92BF4.4030405@linaro.org> <20160209103748.GP11415@e106622-lin> Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org, peterz@infradead.org, vincent.guittot@linaro.org, robh+dt@kernel.org, mark.rutland@arm.com, linux@arm.linux.org.uk, sudeep.holla@arm.com, lorenzo.pieralisi@arm.com, catalin.marinas@arm.com, will.deacon@arm.com, morten.rasmussen@arm.com, dietmar.eggemann@arm.com, broonie@kernel.org From: Steve Muckle X-Enigmail-Draft-Status: N1110 Message-ID: <56BA221D.1030907@linaro.org> Date: Tue, 9 Feb 2016 09:30:05 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20160209103748.GP11415@e106622-lin> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/09/2016 02:37 AM, Juri Lelli wrote: >> I'm still concerned that there's no way to obtain optimal boot time on a >> > heterogeneous system. Either the dynamic benchmarking is enabled, adding >> > 1 sec, or the benchmarking is skipped, and task distribution on the >> > heterogeneous CPUs is determined by the platform's CPU numbering and >> > chance, potentially impacting performance nondeterministically until >> > userspace sets the correct capacity values via sysfs. >> > >> > I believe you tested the impact on boot time of using equal capacity >> > values and saw little difference. I'm wondering though, what was the CPU >> > numbering on that target? >> > > > My targets (Juno and TC2) had big cluster on 1,2 and little on the > remaining cpus. Why do you think this might matter? There's a natural bias in the scheduler AFAIK towards lower-numbered CPUs since they are typically scanned in numerically ascending order. So when all capacities are initially defaulted to be the same I think you'll be more likely to use the lower numbered CPUs. I'd be curious what the performance penalty is on a b.L system where the lowest numbered CPUs are small. I don't have such a target but maybe it's possible to compare booting just with bigs vs just with littles, at least until userspace intializes and a script can bring up the others, which is the same point at which capacities could be properly set. That would give something of an upper bound. > Anyway, IMHO boot time performance is not what we are targeting here, so > I wouldn't be too worried about this particular point. It may not be the most important thing but it is a factor worth considering - as mentioned earlier there are applications where boot time is critical such as automotive. It seems unfortunate that actual performance may be left on the table due to (IMO anyway) a tenuous concern over DT semantics. But it looks like that may just be my position :/ . thanks, Steve