Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757537AbdCUPC0 (ORCPT ); Tue, 21 Mar 2017 11:02:26 -0400 Received: from cloudserver094114.home.net.pl ([79.96.170.134]:58053 "EHLO cloudserver094114.home.net.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755420AbdCUPCZ (ORCPT ); Tue, 21 Mar 2017 11:02:25 -0400 From: "Rafael J. Wysocki" To: Patrick Bellasi Cc: Vincent Guittot , Peter Zijlstra , Linux PM , LKML , Srinivas Pandruvada , Viresh Kumar , Juri Lelli , Joel Fernandes , Morten Rasmussen , Ingo Molnar Subject: Re: [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing frequency of busy CPUs Date: Tue, 21 Mar 2017 15:50:10 +0100 Message-ID: <2491055.bx2pWYvmMQ@aspire.rjw.lan> User-Agent: KMail/4.14.10 (Linux/4.10.0+; KDE/4.14.9; x86_64; ; ) In-Reply-To: <1844525.jBn1oKmyb6@aspire.rjw.lan> References: <4366682.tsferJN35u@aspire.rjw.lan> <20170321143842.GE11054@e110439-lin> <1844525.jBn1oKmyb6@aspire.rjw.lan> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4794 Lines: 100 On Tuesday, March 21, 2017 03:46:07 PM Rafael J. Wysocki wrote: > On Tuesday, March 21, 2017 02:38:42 PM Patrick Bellasi wrote: > > On 21-Mar 15:26, Rafael J. Wysocki wrote: > > > On Tuesday, March 21, 2017 02:37:08 PM Vincent Guittot wrote: > > > > On 21 March 2017 at 14:22, Peter Zijlstra wrote: > > > > > On Tue, Mar 21, 2017 at 09:50:28AM +0100, Vincent Guittot wrote: > > > > >> On 20 March 2017 at 22:46, Rafael J. Wysocki wrote: > > > > > > > > > >> > To work around this issue use the observation that, from the > > > > >> > schedutil governor's perspective, it does not make sense to decrease > > > > >> > the frequency of a CPU that doesn't enter idle and avoid decreasing > > > > >> > the frequency of busy CPUs. > > > > >> > > > > >> I don't fully agree with that statement. > > > > >> If there are 2 runnable tasks on CPU A and scheduler migrates the > > > > >> waiting task to another CPU B so CPU A is less loaded now, it makes > > > > >> sense to reduce the OPP. That's even for that purpose that we have > > > > >> decided to use scheduler metrics in cpufreq governor so we can adjust > > > > >> OPP immediately when tasks migrate. > > > > >> That being said, i probably know why you see such OPP switches in your > > > > >> use case. When we migrate a task, we also migrate/remove its > > > > >> utilization from CPU. > > > > >> If the CPU is not overloaded, it means that runnable tasks have all > > > > >> computation that they need and don't have any reason to use more when > > > > >> a task migrates to another CPU. so decreasing the OPP makes sense > > > > >> because the utilzation is decreasing > > > > >> If the CPU is overloaded, it means that runnable tasks have to share > > > > >> CPU time and probably don't have all computations that they would like > > > > >> so when a task migrate, the remaining tasks on the CPU will increase > > > > >> their utilization and fill space left by the task that has just > > > > >> migrated. So the CPU's utilization will decrease when a task migrates > > > > >> (and as a result the OPP) but then its utilization will increase with > > > > >> remaining tasks running more time as well as the OPP > > > > >> > > > > >> So you need to make the difference between this 2 cases: Is a CPU > > > > >> overloaded or not. You can't really rely on the utilization to detect > > > > >> that but you could take advantage of the load which take into account > > > > >> the waiting time of tasks > > > > > > > > > > I'm confused. What two cases? You only list the overloaded case, but he > > > > > > > > overloaded vs not overloaded use case. > > > > For the not overloaded case, it makes sense to immediately update to > > > > OPP to be aligned with the new utilization of the CPU even if it was > > > > not idle in the past couple of ticks > > > > > > Yes, if the OPP (or P-state if you will) can be changed immediately. If it can't, > > > conditions may change by the time we actually update it and in that case It'd > > > be better to wait and see IMO. > > > > > > In any case, the theory about migrating tasks made sense to me, so below is > > > what I tested. It works, and besides it has a nice feature that I don't need > > > to fetch for the timekeeping data. :-) > > > > > > I only wonder if we want to do this or only prevent the frequency from > > > decreasing in the overloaded case? > > > > > > --- > > > kernel/sched/cpufreq_schedutil.c | 8 +++++--- > > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > > > Index: linux-pm/kernel/sched/cpufreq_schedutil.c > > > =================================================================== > > > --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c > > > +++ linux-pm/kernel/sched/cpufreq_schedutil.c > > > @@ -61,6 +61,7 @@ struct sugov_cpu { > > > unsigned long util; > > > unsigned long max; > > > unsigned int flags; > > > + bool overload; > > > }; > > > > > > static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu); > > > @@ -207,7 +208,7 @@ static void sugov_update_single(struct u > > > if (!sugov_should_update_freq(sg_policy, time)) > > > return; > > > > > > - if (flags & SCHED_CPUFREQ_RT_DL) { > > > + if ((flags & SCHED_CPUFREQ_RT_DL) || this_rq()->rd->overload) { > > > next_f = policy->cpuinfo.max_freq; > > > > Isn't this going to max OPP every time we have more than 1 task in > > that CPU? > > > > In that case it will not fit the case: we have two 10% tasks on that CPU. > > Good point. > > > Previous solution was better IMO, apart from using overloaded instead > > of overutilized (which is not yet there) :-/ > > OK, so the one below works too. Admittedly, we could check the idle condition and the overload flag at the same time, though. Let me try that too. Thanks, Rafael