Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751167AbdFVEGs (ORCPT ); Thu, 22 Jun 2017 00:06:48 -0400 Received: from mail-pg0-f41.google.com ([74.125.83.41]:35576 "EHLO mail-pg0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750847AbdFVEGr (ORCPT ); Thu, 22 Jun 2017 00:06:47 -0400 Date: Thu, 22 Jun 2017 09:36:43 +0530 From: Viresh Kumar To: Morten Rasmussen Cc: Saravana Kannan , Dietmar Eggemann , "linux-kernel@vger.kernel.org" , Linux PM list , Russell King , "linux-arm-kernel@lists.infradead.org" , Greg Kroah-Hartman , Russell King , Catalin Marinas , Will Deacon , Juri Lelli , Vincent Guittot , Peter Zijlstra Subject: Re: [PATCH 2/6] drivers base/arch_topology: frequency-invariant load-tracking support Message-ID: <20170622040643.GB6314@vireshk-i7> References: <20170608075513.12475-1-dietmar.eggemann@arm.com> <20170608075513.12475-3-dietmar.eggemann@arm.com> <5949BE5F.4020502@codeaurora.org> <20170621053735.GR3942@vireshk-i7> <20170621165714.GB2551@e105550-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170621165714.GB2551@e105550-lin.cambridge.arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2570 Lines: 64 On 21-06-17, 17:57, Morten Rasmussen wrote: > It is true that this patch relies on the notifiers, but I don't see how > that prevents us from adding a non-notifier based solution for > fast-switch enabled platforms later? No it doesn't, but I thought it would be better to have a single solution (if possible) for all the cases here. > > I think this patch doesn't really need to go down the notifiers way. > > > > We can do something like this in the implementation of > > topology_get_freq_scale(): > > > > return (policy->cur << SCHED_CAPACITY_SHIFT) / max; > > > > Though, we would be required to take care of policy structure in this > > case somehow. > > This is exactly what this patch implements. Unfortunately we can't be > sure that there is a valid policy data structure where we can read the > information from. Actually there is a way around that. - Revert one of my patches: commit f9f41e3ef99a ("cpufreq: Remove policy create/remove notifiers") - Use those notifiers in init_cpu_capacity_callback() instead of CPUFREQ_NOTIFY and set/reset a local policy pointer. - And this pointer we can use safely/reliably in topology_get_freq_scale(). We may need to use RCU read side protection in topology_get_freq_scale() though, to make sure the local policy pointer isn't getting updated simultaneously. - If the policy pointer isn't set, then we can use SCHED_CAPACITY_SCALE value instead. > Isn't the policy protected by a lock as well? There are locks, but you don't need any to read policy->cur. > Another thing is that I don't think a transition notifier based solution > or any other solution based on the cur/max ratio is really the right way > to go for fast-switching platforms. If we can do very frequent frequency > switching it makes less sense to use the current ratio whenever we > update the PELT averages as the frequency might have changed multiple > times since the last update. So it would make more sense to have an > average ratio instead. > If the platform has HW counters (e.g. APERF/MPERF) that can provide the > ratio then we should of course use those, if not, one solution could be > to let cpufreq track the average frequency for each cpu over a suitable > time window (around one sched period I think). It should be fairly low > overhead to maintain. In the topology driver, we would then choose > whether the scaling factor is provided by the cpufreq average frequency > ratio or the current transition notifier based approach based on the > capabilities of the platform. Hmm, maybe. -- viresh