From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754768AbdCTMvq (ORCPT ); Mon, 20 Mar 2017 08:51:46 -0400 Received: from merlin.infradead.org ([205.233.59.134]:43016 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753559AbdCTMu4 (ORCPT ); Mon, 20 Mar 2017 08:50:56 -0400 Date: Mon, 20 Mar 2017 13:50:09 +0100 From: Peter Zijlstra To: "Rafael J. Wysocki" Cc: Linux PM , LKML , Srinivas Pandruvada , Viresh Kumar , Juri Lelli , Vincent Guittot , Patrick Bellasi , Joel Fernandes , Morten Rasmussen , Ingo Molnar Subject: Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs Message-ID: <20170320125009.nmi3mvrxappjrvgo@hirez.programming.kicks-ass.net> References: <4366682.tsferJN35u@aspire.rjw.lan> <2185243.flNrap3qq1@aspire.rjw.lan> <20170320103645.5jm2rwrx6bndwqmw@hirez.programming.kicks-ass.net> <1846666.Va7CsUJLIx@aspire.rjw.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1846666.Va7CsUJLIx@aspire.rjw.lan> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 20, 2017 at 01:35:12PM +0100, Rafael J. Wysocki wrote: > On Monday, March 20, 2017 11:36:45 AM Peter Zijlstra wrote: > > On Sun, Mar 19, 2017 at 02:34:32PM +0100, Rafael J. Wysocki wrote: > > > From: Rafael J. Wysocki > > > > > > The PELT metric used by the schedutil governor underestimates the > > > CPU utilization in some cases. The reason for that may be time spent > > > in interrupt handlers and similar which is not accounted for by PELT. > > > > > > That can be easily demonstrated by running kernel compilation on > > > a Sandy Bridge Intel processor, running turbostat in parallel with > > > it and looking at the values written to the MSR_IA32_PERF_CTL > > > register. Namely, the expected result would be that when all CPUs > > > were 100% busy, all of them would be requested to run in the maximum > > > P-state, but observation shows that this clearly isn't the case. > > > The CPUs run in the maximum P-state for a while and then are > > > requested to run slower and go back to the maximum P-state after > > > a while again. That causes the actual frequency of the processor to > > > visibly oscillate below the sustainable maximum in a jittery fashion > > > which clearly is not desirable. > > > > > > To work around this issue use the observation that, from the > > > schedutil governor's perspective, CPUs that are never idle should > > > always run at the maximum frequency and make that happen. > > > > > > To that end, add a counter of idle calls to struct sugov_cpu and > > > modify cpuidle_idle_call() to increment that counter every time it > > > is about to put the given CPU into an idle state. Next, make the > > > schedutil governor look at that counter for the current CPU every > > > time before it is about to start heavy computations. If the counter > > > has not changed for over SUGOV_BUSY_THRESHOLD time (equal to 50 ms), > > > the CPU has not been idle for at least that long and the governor > > > will choose the maximum frequency for it without looking at the PELT > > > metric at all. > > > > Why the time limit? > > One iteration appeared to be a bit too aggressive, but honestly I think > I need to check again if this thing is regarded as viable at all. > I don't hate the idea; if we don't hit idle; we shouldn't shift down. I just wonder if we don't already keep a idle-seqcount somewhere; NOHZ and RCU come to mind as things that might already use something like that.