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

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Message-ID: <CAJZ5v0h-Kw8WLJZO5s5NsHTdGC5gomxeXDcbRLFhFoVmm43s9A@mail.gmail.com>
Date: Tue, 27 Jan 2026 17:08:04 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Yaxiong Tian <tianyaxiong@...inos.cn>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>, 
	srinivas.pandruvada@...ux.intel.com, lenb@...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 Mon, Jan 26, 2026 at 3:15 AM Yaxiong Tian <tianyaxiong@...inos.cn> wrote:
>
>
> 在 2026/1/24 04:12, Rafael J. Wysocki 写道:
> > 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.
> Thank you for your explanation. It seems I didn’t read the document
> carefully enough before.
>
> >
> >> 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.
>
> Um, although I've already done it, I agree with your point.
>
> Besides, I don't think this patch is very suitable either, because I
> supposethat on a n system without SMT, the SMT-related|/sys|interface
> should indicate unsupported (though I don't have such a machine).
>
> Moreover, based on the previous discussion, would it be more appropriate
> to use !cpu_smt_possible() instead of !sched_smt_active()? After all,
> the original design was intended for systems without SMT. If you also
> agree with this perspective, I can test it on a machine with SMT.

Yes, please!

Using cpu_smt_possible() instead of sched_smt_active() would be the
most straightforward way to address this, provided that it works.