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

On Sun, Nov 28, 2021 at 09:08:41PM +1000, Nicholas Piggin wrote:

Excerpts from Yury Norov's message of November 28, 2021 1:56 pm:

quoted

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. I'll try to do this.