Messages in this thread | | | Subject | Re: sched: Question about big and little cores system with SMP and EAS | From | Dietmar Eggemann <> | Date | Wed, 16 Jun 2021 15:20:30 +0200 |
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- 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
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