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

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Message-ID: <566bd13f-54b7-42e3-808a-619a58ce05ab@efficios.com>
Date: Mon, 25 Aug 2025 14:02:10 -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 2025-08-23 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 ?

Thanks,

Mathieu

> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>   kernel/rseq.c |   11 ++++++-----
>   1 file changed, 6 insertions(+), 5 deletions(-)
> 
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -464,11 +464,12 @@ void __rseq_handle_notify_resume(struct
>   		t->rseq_event_pending = false;
>   	}
>   
> -	if (IS_ENABLED(CONFIG_DEBUG_RSEQ) || event) {
> -		ret = rseq_ip_fixup(regs, event);
> -		if (unlikely(ret < 0))
> -			goto error;
> -	}
> +	if (!IS_ENABLED(CONFIG_DEBUG_RSEQ) && !event)
> +		return;
> +
> +	ret = rseq_ip_fixup(regs, event);
> +	if (unlikely(ret < 0))
> +		goto error;
>   
>   	if (unlikely(rseq_update_cpu_node_id(t)))
>   		goto error;
> 

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