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

[Michał Mirosław]

On Sat, Nov 27, 2021 at 07:56:55PM -0800, Yury Norov wrote:
> 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.

Having slept on it I have more structured thoughts:

First, I like substituting bitmap_empty/full where possible - I think
the change stands on its own, so could be split and sent as is.

I don't like the proposed API very much. One problem is that it hides
the comparison operator and makes call sites less readable:

	bitmap_weight(...) > N

becomes:

	bitmap_weight_gt(..., N)

and:
	bitmap_weight(...) <= N

becomes:

	bitmap_weight_lt(..., N+1)
or:
	!bitmap_weight_gt(..., N)

I'd rather see something resembling memcmp() API that's known enough
to be easier to grasp. For above examples:

	bitmap_weight_cmp(..., N) > 0
	bitmap_weight_cmp(..., N) <= 0
	...

This would also make the implementation easier in not having to
copy and paste the code three times. Could also use a simple
optimization reducing code size:

#include <linux/overflow.h>

int bitmap_weight_cmp(long *bits, size_t nbits, size_t cmp)
{
	for (size_t i = 0; i < nbits / BITS_PER_LONG; ++i, ++bits)
		if (check_sub_overflow(cmp, popcount(*bits), &cmp))
			return 1;

	nbits %= BITS_PER_LONG;
	if (nbits && check_sub_overflow(cmp,
			popcount(*bits & GENMASK(nbits)), &cmp))
		return 1;

	return cmp ? -1 : 0;
}

Best Regards
Michał Mirosław