Re: [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage

Nicholas Piggin
2021-11-28T11:10:54Z

Excerpts from Yury Norov's message of November 28, 2021 1:56 pm:
> In many cases people use bitmap_weight()-based functions like this:
>
> if (num_present_cpus() > 1)
> do_something();
>
> This may take considerable amount of time on many-cpus machines because
> num_present_cpus() will traverse every word of underlying cpumask
> unconditionally.
>
> We can significantly improve on it for many real cases if stop traversing
> the mask as soon as we count present cpus to any number greater than 1:
>
> if (num_present_cpus_gt(1))
> do_something();
>
> To implement this idea, the series adds bitmap_weight_{eq,gt,le}
> functions together with corresponding wrappers in cpumask and nodemask.

There would be no change to callers if you maintain counters like what
is done for num_online_cpus() today. Maybe some fixes to arch code that
does not use set_cpu_possible() etc APIs required, but AFAIKS it would
be better to fix such cases anyway.

Thanks,
Nick