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

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Message-ID: <20231218161100.GA7839@ranerica-svr.sc.intel.com>
Date: Mon, 18 Dec 2023 08:11:00 -0800
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Bagas Sanjaya <bagasdotme@...il.com>,
	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>
Subject: Re: Fwd: Intel hybrid CPU scheduler always prefers E cores

On Tue, Nov 28, 2023 at 03:02:25PM +0100, Peter Zijlstra wrote:
> 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?

Sorry for the late reply. This email was buried in a ton of email. To
complete report here, Srinivas helped to debug the issue. The problem is
that the computer in question lacks the necessary ACPI support to use ITMT.

A new firmware release appears to have solved the issue.

Thanks and BR,
Ricardo