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

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Message-ID: <87ldmqwgjc.ffs@tglx>
Date: Sun, 07 Sep 2025 15:14:47 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: "Rafael J. Wysocki" <rafael@...nel.org>
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 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.