Date: Fri, 4 Jul 2014 17:55:52 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        peterz@infradead.org, mingo@kernel.org, rjw@rjwysocki.net,
        vincent.guittot@linaro.org, daniel.lezcano@linaro.org,
        preeti@linux.vnet.ibm.com, Dietmar.Eggemann@arm.com, pjt@google.com
Subject: Re: [RFCv2 PATCH 00/23] sched: Energy cost model for energy-aware
 scheduling
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Hi Morten,

On Thu, Jul 03, 2014 at 05:25:47PM +0100, Morten Rasmussen wrote:
> This is an RFC and there are some loose ends that have not been
> addressed here or in the code yet. The model and its infrastructure is
> in place in the scheduler and it is being used for load-balancing
> decisions. It is used for the select_task_rq_fair() path for
> fork/exec/wake balancing and to guide the selection of the source cpu
> for periodic or idle balance.

IMHO, the series is on the right direction for addressing the energy
aware scheduling (very complex) problem. But I have some high level
comments below.

> However, the main ideas and the primary focus of this RFC: The energy
> model and energy_diff_{load, task, cpu}() are there.
> 
> Due to limitation 1, the ARM TC2 platform (2xA15+3xA7) was setup to
> disable frequency scaling and set frequencies to eliminate the
> big.LITTLE performance difference. That basically turns TC2 into an SMP
> platform where a subset of the cpus are less energy-efficient.
> 
> Tests using a synthetic workload with seven short running periodic
> tasks of different size and period, and the sysbench cpu benchmark with
> five threads gave the following results:
> 
> cpu energy*	short tasks	sysbench
> Mainline	100		100
> EA		 49		 99
> 
> * Note that these energy savings are _not_ representative of what can be
> achieved on a true SMP platform where all cpus are equally 
> energy-efficient. There should be benefit for SMP platforms as well, 
> however, it will be smaller.

My impression (and I may be wrong) is that you get bigger energy saving
on a big.LITTLE vs SMP system exactly because of the asymmetry in power
consumption. The algorithm proposed here ends up packing small tasks on
the little CPUs as they are more energy efficient (which is the correct
thing to do but I wonder what results you would get with 3xA7 vs
2xA7+1xA15).

For a symmetric system where all CPUs have the same energy model you
could end up with several small threads balanced equally across the
system. The only way the scheduler could avoid a CPU is if it somehow
manages to get into a deeper idle state (and energy_diff_task() would
show some asymmetry). But this wouldn't happen without the scheduler
first deciding to leave that CPU idle for longer.

Could this be addressed by making the scheduler more "proactive" and,
rather than just looking at the current energy diff, guesstimate what it
would be if not placing a task at all on the CPU? If for example there
is no other task running on that CPU, could energy_diff_task() take into
account the next deeper C-state rather than just the current one? This
way we may be able to achieve more packing even on fully symmetric
systems and allow CPUs to go into deeper sleep states.

Thanks.

-- 
Catalin
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