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

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Date:   Tue, 28 Nov 2023 15:02:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bagas Sanjaya <bagasdotme@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Regressions <regressions@...ts.linux.dev>,
        Linux Power Management <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ramses VdP <ramses@...l-founded.dev>,
        ricardo.neri-calderon@...ux.intel.com
Subject: Re: Fwd: Intel hybrid CPU scheduler always prefers E cores

On Tue, Nov 28, 2023 at 08:22:27PM +0700, Bagas Sanjaya wrote:
> Hi,
> 
> I come across an interesting bug report on Bugzilla [1]. The reporter
> wrote:

Thanks for forwarding, what happend in bugzilla staysi in bugzilla etc..

Did you perchance Cc the reporter?

> > 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?

AFAIK that is supposed to be steered by the ITMT muck and I don't think
we changed that.

Ricardo?

> 
> Thanks.
> 
> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=218195
> 
> -- 
> An old man doll... just what I always wanted! - Clara