On Fri, Nov 11, 2022 at 01:42:29PM +0200, Andy Shevchenko wrote:
> On Thu, Nov 10, 2022 at 08:00:26PM -0800, Yury Norov wrote:
> > The function finds Nth set CPU in a given cpumask starting from a given
> > node.
> >
> > Leveraging the fact that each hop in sched_domains_numa_masks includes the
> > same or greater number of CPUs than the previous one, we can use binary
> > search on hops instead of linear walk, which makes the overall complexity
> > of O(log n) in terms of number of cpumask_weight() calls.
>
> ...
>
> > +int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
> > +{
> > + unsigned int first = 0, mid, last = sched_domains_numa_levels;
> > + struct cpumask ***masks;
>
> *** ?
> Hmm... Do we really need such deep indirection?
It's 2d array of pointers, so - yes.
> > + int w, ret = nr_cpu_ids;
> > +
> > + rcu_read_lock();
> > + masks = rcu_dereference(sched_domains_numa_masks);
> > + if (!masks)
> > + goto out;
> > +
> > + while (last >= first) {
> > + mid = (last + first) / 2;
> > +
> > + if (cpumask_weight_and(cpus, masks[mid][node]) <= cpu) {
> > + first = mid + 1;
> > + continue;
> > + }
> > +
> > + w = (mid == 0) ? 0 : cpumask_weight_and(cpus, masks[mid - 1][node]);
>
> See below.
>
> > + if (w <= cpu)
> > + break;
> > +
> > + last = mid - 1;
> > + }
>
> We have lib/bsearch.h. I haven't really looked deeply into the above, but my
> gut feelings that that might be useful here. Can you check that?
Yes we do. I tried it, and it didn't work because nodes arrays are
allocated dynamically, and distance between different pairs of hops
for a given node is not a constant, which is a requirement for
bsearch.
However, distance between hops pointers in 1st level array should be
constant, and we can try feeding bsearch with it. I'll experiment with
bsearch for more.
> > + ret = (mid == 0) ?
> > + cpumask_nth_and(cpu - w, cpus, masks[mid][node]) :
> > + cpumask_nth_and_andnot(cpu - w, cpus, masks[mid][node], masks[mid - 1][node]);
>
> You can also shorten this by inversing the conditional:
>
> ret = mid ? ...not 0... : ...for 0...;
Yep, why not.
> > +out:
>
> out_unlock: ?
Do you think it's better?
> > + rcu_read_unlock();
> > + return ret;
> > +}
>
> --
> With Best Regards,
> Andy Shevchenko
>
