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

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Message-ID: <1cdacbf9-9564-4025-a5a8-777211a1fc2d@efficios.com>
Date: Thu, 11 Sep 2025 13:15:09 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, 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 2025-09-11 12:06, Thomas Gleixner wrote:
> 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.
Note that this concerns code that documents an explicit " & " as
intentional to spare a branch, which led me to assume that optimizing
it was important.

Thanks,

Mathieu

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