Re: [RFC PATCH 0/3] CFS idle injection

From: Punit Agrawal <punit.agrawal@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Eduardo Valentin <edubezval@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Arjan van de Ven <arjan@linux.intel.com>,
	Paul Turner <pjt@google.com>, Len Brown <len.brown@intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Andi Kleen <andi.kleen@intel.com>,
	Rafael Wysocki <rafael.j.wysocki@intel.com>
Subject: Re: [RFC PATCH 0/3] CFS idle injection
Date: Fri, 06 Nov 2015 16:50:15 +0000	[thread overview]
Message-ID: <9hhvb9f843c.fsf@e105922-lin.cambridge.arm.com> (raw)
In-Reply-To: <20151105101218.GB3604@twins.programming.kicks-ass.net> (Peter Zijlstra's message of "Thu, 5 Nov 2015 11:12:18 +0100")

Peter Zijlstra <peterz@infradead.org> writes:

> People, trim your emails!
>
> On Wed, Nov 04, 2015 at 08:58:30AM -0800, Jacob Pan wrote:
>
>> > I also like #2 too. Specially now that it is not limited to a specific
>> > platform. One question though, could you still keep the cooling device
>> > support of it? In some systems, it might make sense to enable /
>> > disable idle injections based on temperature.
>
>> One of the key difference between 1 and 2 is that #2 is open loop
>> control, since we don't have CPU c-states info baked into scheduler. 
>
> _yet_, there's people working on that. The whole power aware scheduling
> stuff needs that.
>  
>> To close the loop, perhaps we can export some internal APIs to the
>> thermal subsystem then the thermal governors can pick the condition to
>> inject idle.
>
> I would much rather that all be part of the power aware stuff, such that
> the scheduler itself is aware of thermal limits and can migrate load
> away if needed.

I was wondering if we could use cpu capacity as the interface between
the thermal sub-system and the scheduler. This would be better than
dealing with frequency caps and idle injection percentages directly in
the scheduler.

We've been playing with making the scheduler respect capacity caps due
to thermal constraints and have tasks migrated away to less capped
cores.

It would be great if in addition to the frequency caps, we could add
idle injection to the arsenal. This would allow building policies on top
such as -

* pure idle injection where frequency capping is unsuitable (or
  unavailable)
* a smooth continuum of capacities using a combination of frequency and
  capacity capping
* idle injection once frequencies have been capped to the lowest
  feasible values (as suggested in the cover letter)

One question about the implementation in these patches - should the
implementation hook into pick_next_task in core instead of CFS? Higher
priority tasks might get in the way of idle injection.

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