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Subject: Re: [RFC][PATCH v5 00/14] sched: packing tasks
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From: Catalin Marinas <catalin.marinas@arm.com>
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Date: Mon, 11 Nov 2013 23:43:23 +0000
Cc: Morten Rasmussen <Morten.Rasmussen@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
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To: Arjan van de Ven <arjan@linux.intel.com>
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On 11 Nov 2013, at 19:26, Arjan van de Ven <arjan@linux.intel.com> wrote:
> On 11/11/2013 10:31 AM, Catalin Marinas wrote:
>> I agree, we can't rely on the requested C-state but the_actual_  state
>> and this means querying the hardware driver. Can we abstract this via
>> some interface which provides the cost of waking up a CPU? This could
>> take into account the state of the other CPUs in the cluster and the
>> scheduler is simply concerned with the wake-up costs.
> 
> can you even query this without actually waking up the cpu and asking ???

Even if you don’t have additional hardware to query the state of a CPU
without waking it up, we could have a per-CPU variable storing the
actual C-states as selected by the arch backend.  This doesn’t need to
be precise but lets say only 90% accurate would probably be enough for
the scheduler.

Catalin