linux-kernel - Re: sched: Question about big and little cores system with SMP and EAS

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Message-ID: <c81ac071-c648-54ac-72ad-2dab0fa1dd4b@arm.com>
Date:   Wed, 16 Jun 2021 15:20:30 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ley Foon Tan <lftan.linux@...il.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Quentin Perret <qperret@...gle.com>
Subject: Re: sched: Question about big and little cores system with SMP and
 EAS

- Quentin Perret <quentin.perret@....com>
+ Quentin Perret <qperret@...gle.com>

On 16/06/2021 13:39, Peter Zijlstra wrote:
> On Wed, Jun 16, 2021 at 07:29:26PM +0800, Ley Foon Tan wrote:
>> Hi all
>>
>> Would like to ask the experts here regarding the Symmetric
>> Multi-Processing mode (SMP) with Energy aware scheduler (EAS) support
>> on the big + little cores system.
> 
> And the you ask a question unrelated to either Symmetric MP or EAS :-)
> 
>> Hardware system:
>> Big and little cores have almost the same ISA, but the big core has
>> some extension instructions that little core doesn't have.
> 
> That problem is unrelated to big.Little / EAS, also by definition that
> is not SMP seeing how the 'S' is a blatant lie.
> 
> The simplest solution is to simply disallow usage of the extended ISA
> and force mandate the common subset. The complicated answer is something
> along the lines of:
> 
>   https://lkml.kernel.org/r/20210608180313.11502-1-will@kernel.org

We don't encourage asymmetric ISA extensions for EAS*/CAS** on
big.Little systems.
It would be simply a nightmare to schedule tasks on such systems.

The exception to this is the 'asymmetric 32-bit Soc' to support 32bit
legacy Apps. The nightmare for scheduling is reduced in this case to CPU
affinity, something the task scheduler has to live with already today.
(+ DL admission control for 32bit tasks).

*  Documentation/scheduler/sched-energy.rst
** Documentation/scheduler/sched-capacity.rst