Re: [PATCH v2 0/4] cpumask: improve on cpumask_local_spread() locality

Valentin Schneider
2022-11-17T12:24:56Z

On 15/11/22 10:32, Yury Norov wrote:
> On Tue, Nov 15, 2022 at 05:24:56PM +0000, Valentin Schneider wrote:
>>
>> Is this meant as a replacement for [1]?
>
> No. Your series adds an iterator, and in my experience the code that
> uses iterators of that sort is almost always better and easier to
> understand than cpumask_nth() or cpumask_next()-like users.
>
> My series has the only advantage that it allows keep existing codebase
> untouched.
>

Right

>> I like that this is changing an existing interface so that all current
>> users directly benefit from the change. Now, about half of the users of
>> cpumask_local_spread() use it in a loop with incremental @i parameter,
>> which makes the repeated bsearch a bit of a shame, but then I'm tempted to
>> say the first point makes it worth it.
>>
>> [1]: https://lore.kernel.org/all/20221028164959.1367250-1-vschneid@redhat.com/
>
> In terms of very common case of sequential invocation of local_spread()
> for cpus from 0 to nr_cpu_ids, the complexity of my approach is n * log n,
> and your approach is amortized O(n), which is better. Not a big deal _now_,
> as you mentioned in the other email. But we never know how things will
> evolve, right?
>
> So, I would take both and maybe in comment to cpumask_local_spread()
> mention that there's a better alternative for those who call the
> function for all CPUs incrementally.
>

Ack, sounds good.

> Thanks,
> Yury