linux-kernel - Re: [patch V2 08/37] rseq: Avoid CPU/MM CID updates when no event pending

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Message-ID: <831f465b-3fe2-41a4-a50b-437f2e028864@efficios.com>
Date: Thu, 4 Sep 2025 13:20:47 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
 Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 08/37] rseq: Avoid CPU/MM CID updates when no event
 pending

On 2025-09-02 09:41, Thomas Gleixner wrote:
> On Mon, Aug 25 2025 at 14:02, Mathieu Desnoyers wrote:
>> On 2025-08-23 12:39, Thomas Gleixner wrote:
>>> There is no need to update these values unconditionally if there is no
>>> event pending.
>>
>> I agree with this change.
>>
>> On a related note, I wonder if arch/powerpc/mm/numa.c:
>> find_and_update_cpu_nid() should set the rseq_event pending bool to true
>> for each thread in the system ?
> 
> What for? That's the hotplug path, which establishes the CPU to node
> relationship for newly added CPUs and their [hyper]threads _before_
> those CPUs are able to run anything.

Agreed. Thanks!

Mathieu

> 
> Thanks,
> 
>          tglx

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com