linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Disable SMT when hybrid systems are enabled

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Message-ID: <CAJZ5v0ipPwmBe0bxKBJc203ug6ENzT6OEaOruyL=vUw6NWMZfQ@mail.gmail.com>
Date: Fri, 23 Jan 2026 21:12:37 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Yaxiong Tian <tianyaxiong@...inos.cn>
Cc: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>, 
	srinivas.pandruvada@...ux.intel.com, lenb@...nel.org, rafael@...nel.org, 
	viresh.kumar@...aro.org, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: intel_pstate: Disable SMT when hybrid systems
 are enabled

On Fri, Jan 23, 2026 at 2:20 AM Yaxiong Tian <tianyaxiong@...inos.cn> wrote:
>
>
> 在 2026/1/22 23:55, Ricardo Neri 写道:
> > On Mon, Jan 19, 2026 at 03:41:18PM +0800, Yaxiong Tian wrote:
> >> When hwp_is_hybrid && !sched_smt_active(), the driver enables
> >> hybrid_capacity_scale and disables ITMT. According to the original code
> >> logic, these related actions are one-time operations; therefore, I
> >> believe the original design intent did not support dynamic runtime
> >> switching.
> >>
> >> However, SMT can be toggled via related interfaces in /sys. When SMT is
> >> enabled, the system is no longer hybrid, and the original settings become
> >> incorrect.
> > Indeed I was able to enable SMT siblings:
> >
> >       $ echo on > /sys/devices/system/cpu/smt
> >       $ echo 1 > /sys/devices/system/cpu/cpu1/online
> >
> >> To resolve this confusion, permanently disable SMT by calling
> >> cpuhp_smt_disable().
> > IMHO, the user should be able to enable SMT back if she or he chooses to. Instead,
> > the sched domains should be rebuilt with asym packing and without asymmetric
> > capacity.
> Yes, I also agree with this viewpoint.

I don't agree though.

> I think a better solution is to
> place it within cpufreq online for heterogeneous judgment and to clearly
> address sched domain rebuilt and asymmetry issues.

Maybe in theory, but in practice the EAS-related code in intel_pstate
really only targets systems with no SMT.

While it is possible to use it on SMT systems with SMT disabled, I
really wouldn't recommend doing that beyond debug/diagnostics and no,
it is not sufficient to disable stuff when SMT is re-enabled, you'd
basically need to unregister the driver and register it from scratch
in that case.

> I ran some tests the
> day before yesterday and encountered a few locking problems. I will
> publish the patch a bit later.

Given the complexity involved, I don't think it is worth the effort.

I can apply this patch though.