linux-kernel - Re: [patch V4 26/36] rseq: Optimize event setting

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Message-ID: <871podrn2i.ffs@tglx>
Date: Thu, 11 Sep 2025 18:06:13 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Michael Jeanson <mjeanson@...icios.com>, 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 V4 26/36] rseq: Optimize event setting

On Thu, Sep 11 2025 at 10:03, Mathieu Desnoyers wrote:
> On 2025-09-08 17:32, Thomas Gleixner wrote:
>>   static inline void rseq_handle_notify_resume(struct pt_regs *regs)
>>   {
>> -	if (current->rseq.event.has_rseq)
>> +	/* '&' is intentional to spare one conditional branch */
>> +	if (current->rseq.event.sched_switch & current->rseq.event.has_rseq)
>
> I wonder.. except for the corner case of rseq unregistration,
> when can we have sched_switch set but not has_rseq ?
>
> We could remove a load from the fast path and the AND if we
> clear the sched_switch flag on rseq unregistration.

We probably could. Though I doubt it matters much and I opted for
correctness instead of premature optimization.

Thanks,

        tglx