Re: irq_pipeline: likely/unlikely macro usage

On 2/16/26 13:51, Jan Kiszka wrote:
> On 16.02.26 13:26, Tobias Schaffner wrote:
>> Hi all,
>>
>> while staring at the irq_pipelining implementation, I noticed that there
>> are some locations that use the unlikely macro for the OOB fast path,
>> e.g., [1].
>>
>> Could someone explain why we are optimizing for the in-band path here?
>> Although the in-band path is indeed more likely, I would have expected
>> that we would want to optimize for the OOB path anyway, since this is
>> where the saved cycles really matter.
>>
> 
> Likely/unlikely optimizations are generally tricky. They are hints which
> might even be ignored by concrete archs or in concrete situations.
> 
> If they lead to consistently higher latency for oob, I would agree that
> we should lean to that path instead. However, if they reduce the average
> latency for inband and practically do not increase the worst-case for
> oob, it might be a better decision to take that performance optimization
> because the worst-case latency is what matters for oob more.
> 
> But it remains a fair question whether we can answer that question
> whenever we added [un]likely... Did you find inconsistencies in their
> usage as well?

No, it does not look inconsistent to me. It looks like the more frequent 
path is consistently chosen. And I wondered if this is correct.

Best,
Tobias

> Jan
> 
>> Best,
>> Tobias
>>
>> [1] https://gitlab.com/Xenomai/linux-dovetail/-/blob/v5.10.y-dovetail/
>> arch/x86/kernel/irq_pipeline.c?ref_type=heads#L260
>>
>