linux-kernel - Fwd: Intel hybrid CPU scheduler always prefers E cores

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Date:   Tue, 28 Nov 2023 20:22:27 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Regressions <regressions@...ts.linux.dev>,
        Linux Power Management <linux-pm@...r.kernel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ramses VdP <ramses@...l-founded.dev>
Subject: Fwd: Intel hybrid CPU scheduler always prefers E cores

Hi,

I come across an interesting bug report on Bugzilla [1]. The reporter
wrote:

> I am running an intel alder lake system (Core i7-1260P), with a mix of P and E cores.
> 
> Since Linux 6.6, and also on the current 6.7 RC, the scheduler seems to have a strong preference for the E cores, and single threaded workloads are consistently scheduled on one of the E cores.
> 
> With Linux 6.4 and before, when I ran a single threaded CPU-bound process, it was scheduled on a P core. With 6.5, it seems that the choice of P or E seemed rather random.
> 
> I tested these by running "stress" with different amounts of threads. With a single thread on Linux 6.6 and 6.7, I always have an E core at 100% and no load on the P cores. Starting from 3 threads I get some load on the P cores as well, but the E cores stay more heavily loaded.
> With "taskset" I can force a process to run on a P core, but clearly it's not very practical to have to do CPU scheduling manually.
> 
> This severely affects single-threaded performance of my CPU since the E cores are considerably slower. Several of my workflows are now a lot slower due to them being single-threaded and heavily CPU-bound and being scheduled on E cores whereas they would run on P cores before.
> 
> I am not sure what the exact desired behaviour is here, to balance power consumption and performance, but currently my P cores are barely used for single-threaded workloads.
> 
> Is this intended behaviour or is this indeed a regression? Or is there perhaps any configuration that I should have done from my side? Is there any further info that I can provide to help you figure out what's going on?

PM and scheduler people, is this a regression or works as intended?

Thanks.

[1]: https://bugzilla.kernel.org/show_bug.cgi?id=218195

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