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

Mathieu Desnoyers
2025-09-11T17:15:12Z

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.