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Date: Fri, 31 Mar 2017 08:31:26 +0100
From: Juri Lelli <juri.lelli@arm.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
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Subject: Re: [RFD PATCH 4/5] sched/cpufreq_schedutil: always consider all
 CPUs when deciding next freq
Message-ID: <20170331073126.GK19929@e106622-lin>
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On 30/03/17 22:13, Rafael J. Wysocki wrote:
> On Thu, Mar 30, 2017 at 10:58 AM, Juri Lelli <juri.lelli@arm.com> wrote:
> > Hi,
> 
> Hi,
> 
> > On 30/03/17 00:41, Rafael J. Wysocki wrote:
> >> On Friday, March 24, 2017 02:08:59 PM Juri Lelli wrote:
> >> > No assumption can be made upon the rate at which frequency updates get
> >> > triggered, as there are scheduling policies (like SCHED_DEADLINE) which
> >> > don't trigger them so frequently.
> >> >
> >> > Remove such assumption from the code.
> >>
> >> But the util/max values for idle CPUs may be stale, no?
> >>
> >
> > Right, that might be a problem. A proper solution I think would be to
> > remotely update such values for idle CPUs, and I believe Vincent is
> > working on a patch for that.
> >
> > As mid-term workarounds, changing a bit the current one, come to my
> > mind:
> >
> >  - consider TICK_NSEC (continue) only when SCHED_CPUFREQ_DL is not set
> >  - remove CFS contribution (without triggering a freq update) when a CPU
> >    enters IDLE; this might not work well, though, as we probably want
> >    to keep in blocked util contribution for a bit
> >
> > What you think is the way to go?
> 
> Well, do we want SCHED_DEADLINE util contribution to be there even for
> idle CPUs?
> 

DEADLINE util contribution is removed, even if the CPU is idle, by the
reclaiming mechanism when we know (applying GRUB algorithm rules [1])
that it can't be used anymore by a task (roughly speaking). So, we
shouldn't have this problem in the DEADLINE case.

[1] https://marc.info/?l=linux-kernel&m=149029880524038