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

Yury Norov
2022-11-30T01:47:38Z

On Mon, Nov 28, 2022 at 08:39:24AM +0200, Tariq Toukan wrote:
>
>
> On 11/17/2022 2:23 PM, Valentin Schneider wrote:
> > 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.
> >
>
> Good.
> Is a respin needed, to add the comment mentioned above?

If you think it's worth the effort.