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

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Message-ID: <YMni2eclmqf05cL4@hirez.programming.kicks-ass.net>
Date:   Wed, 16 Jun 2021 13:39:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ley Foon Tan <lftan.linux@...il.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Quentin Perret <quentin.perret@....com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: sched: Question about big and little cores system with SMP and
 EAS

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