Re: [RFD PATCH 4/5] sched/cpufreq_schedutil: always consider all CPUs when deciding next freq

[Juri Lelli <juri.lelli@arm.com>]

On 31/03/17 11:03, Rafael J. Wysocki wrote:
> On Fri, Mar 31, 2017 at 9:31 AM, Juri Lelli <juri.lelli@arm.com> wrote:
> > 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
> 
> OK
> 
> Why don't you store the contributions from DL and CFS separately, then
> (say, as util_dl, util_cfs, respectively) and only discard the CFS one
> if delta_ns > TICK_NSEC?

Sure, this should work as well. I'll try this approach for next version.

Thanks,

- Juri