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

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Message-ID: <87ms7d0y5a.ffs@tglx>
Date: Tue, 02 Sep 2025 15:41:53 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 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 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.

Thanks,

        tglx