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

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Date:	Tue, 12 Apr 2016 16:29:06 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Steve Muckle <steve.muckle@...aro.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 11:20 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 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.


This is rather fundamental.

For example, if you look at cpufreq_update_util(), it does this:

data = rcu_dereference_sched(*this_cpu_ptr(&cpufreq_update_util_data));

meaning that it will run the current CPU's utilization update
callback.  Of course, that won't work cross-CPU, because in principle
different CPUs may use different governors and therefore different
util update callbacks.

If you want to do remote updates, I guess that will require an
irq_work to run the update on the target CPU, but then you'll probably
want to neglect the rate limit on it as well, so it looks like a
"need_update" flag in struct update_util_data will be useful for that.

I think I can prototype something along these lines, but can you
please tell me more about the case you have in mind?

Thanks,
Rafael