linux-kernel - Re: [RFC][PATCH v5 00/14] sched: packing tasks

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Message-Id: <42638CC1-ACC7-4330-A4F4-D78C88BE8155@arm.com>
Date:	Tue, 12 Nov 2013 23:14:05 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Tony Luck <tony.luck@...el.com>,
	"alex.shi@...el.com" <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"l.majewski@...sung.com" <l.majewski@...sung.com>,
	Jonathan Corbet <corbet@....net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [RFC][PATCH v5 00/14] sched: packing tasks

On 12 Nov 2013, at 16:48, Arjan van de Ven <arjan@...ux.intel.com> wrote:
> On 11/11/2013 10:18 AM, Catalin Marinas wrote:
>> The ordering is based on the actual C-state, so a simple way is to wake
>> up the CPU in the shallowest C-state. With asymmetric configurations
>> (big.LITTLE) we have different costs for the same C-state, so this would
>> come in handy.
> 
> btw I was considering something else; in practice CPUs will be in the deepest state..
> ... at which point I was going to go with some other metrics of what is best from a platform level

I agree, other metrics are needed.  The problem is that we currently
only have (relatively, guessed from the target residency) the cost of
transition from a C-state to a P-state (for the latter, not sure which).
But we don’t know what the power (saving) on that C-state is nor the one 
at a P-state (and vendors reluctant to provide such information). So the 
best the scheduler can do is optimise the wake-up cost and blindly assume 
that deeper C-state on a CPU is more efficient than lower P-states on two 
other CPUs (or the other way around).

If we find a good use for such metrics in the scheduler, I think the
vendors would be more open to providing at least some relative (rather
than absolute) numbers.

Catalin--
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