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

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Message-ID: <CAJZ5v0gW+A-eyckySFrHc7=Qr9URdRX6NqvPgkq4gZEvs_uBWg@mail.gmail.com>
Date: Thu, 11 Sep 2025 12:31:36 +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 Sun, Sep 7, 2025 at 3:14 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Fri, Sep 05 2025 at 22:56, Rafael J. Wysocki wrote:
> > 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. :-)
>
> Correct.

So are you going to send a patch or do you want me to do something?

>From a user standpoint, this issue is a regression in 6.16, so it
would be good to address it before final 6.17.