linux-kernel - Re: [PATCH v1] cpu: Add missing check to cpuhp_smt

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Message-ID: <CAJZ5v0htmEeivbQaumRc7zw_Zx68GpUy98ksA9L42LupjO6tWA@mail.gmail.com>
Date: Fri, 5 Sep 2025 22:56:43 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Linux PM <linux-pm@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Christian Loehle <christian.loehle@....com>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v1] cpu: Add missing check to cpuhp_smt_enable()

On Fri, Sep 5, 2025 at 10:47 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Fri, Sep 05 2025 at 15:27, Rafael J. Wysocki wrote:
> > On Fri, Sep 5, 2025 at 3:13 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >> Well, manual online can be used for onlining the secondary thread of a
> >> core where the primary thread is offline, so this is technically
> >> possible already.
> >>
> >> > Something like the completely untested below.
> >>
> >> So given the above, shouldn't topology_is_core_online() check if any
> >> thread in the given core is online?
> >
> > Besides, this would cause the siblings of offline SMT threads to be
> > skipped while enabling SMT via sysfs (using
> > /sys/devices/system/cpu/smt/control), but I'm not sure if this is the
> > expectation in the field today.  The current behavior is to online all
> > secondary SMT threads (and more, but that part is quite arguably
> > broken).
>
> It is broken, because the initial logic is to bring up primary threads
> unconditionally and then refuse to bring up sibling threads.
>
> With "maxcpus=xxx" this obviously limits the amount of primary threads,
> so there is arguably no point to online any of the related secondary
> threads of them.
>
> The initial implementation was naively making that assumption, but the
> core check which was added due to PPC made this actually correct.
>
> It just did not snap with me back then, but it's actually the correct
> thing to do, no?

It would at least be consistent with the existing PPC behavior. :-)