linux-kernel - Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load

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Date:	Mon, 11 Apr 2016 23:20:55 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg()

On Mon, Apr 11, 2016 at 9:28 PM, Steve Muckle <steve.muckle@...aro.org> wrote:
> Hi Rafael,
>
> On 04/01/2016 02:20 AM, Peter Zijlstra wrote:
>>> > My thinking was in CFS we get rid of the (cpu == smp_processor_id())
>>> > condition for calling the cpufreq hook.
>>> >
>>> > The sched governor can then calculate utilization and frequency required
>>> > for cpu. If (cpu == smp_processor_id()), the update is processed
>>> > normally. If (cpu != smp_processor_id()) and the new frequency is higher
>>> > than cpu's Fcur, the sched gov IPIs cpu to continue running the update
>>> > operation. Otherwise, the update is dropped.
>>> >
>>> > Does that sound plausible?
>>
>> Can be done I suppose..
>
> Currently we drop schedutil updates for a target CPU which do not occur
> on that CPU.
>
> Is this solely due to platforms which must run the cpufreq driver on the
> target CPU?

The current code assumes that the CPU running the update will always
be the one that gets updated.  Anything else would require extra
synchronization.

> Are there also shared cpufreq policies where the driver needs to run on
> any CPU in the affected policy/freq domain?

Yes, there are, AFAICS, but drivers are expected to cope with that (if
I understand the question correctly).

Thanks,
Rafael