Re: Fwd: Intel hybrid CPU scheduler always prefers E cores

[Tim Chen]

On Tue, 2023-11-28 at 20:22 +0700, Bagas Sanjaya wrote:
> 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
> 

I have noticed that the current code sometimes is quite trigger happy
moving tasks off P-core, whenever there are more than 2 tasks on a core.
Sometimes, Short running house keeping tasks
could disturb the running task on P-core as a result.

Can you try the following patch?  On my Alder Lake system, I see as I add single
threaded tasks, they first run on P-cores, then followed by E-cores with this
patch on 6.6.

Tim