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From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Ley Foon Tan <lftan.linux@gmail.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, Quentin Perret <qperret@google.com>
Subject: Re: sched: Question about big and little cores system with SMP and EAS
Date: Wed, 16 Jun 2021 15:20:30 +0200	[thread overview]
Message-ID: <c81ac071-c648-54ac-72ad-2dab0fa1dd4b@arm.com> (raw)
In-Reply-To: <YMni2eclmqf05cL4@hirez.programming.kicks-ass.net>

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

  reply	other threads:[~2021-06-16 13:20 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-16 11:29 sched: Question about big and little cores system with SMP and EAS Ley Foon Tan
2021-06-16 11:39 ` Peter Zijlstra
2021-06-16 13:20   ` Dietmar Eggemann [this message]
2021-06-17  5:00     ` Ley Foon Tan
2021-06-17  7:51       ` Morten Rasmussen

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