linux-kernel - Re: [PATCH 0/9] lib/bitmap: optimize bitmap

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Message-Id: <1638096766.3elxdzb8ly.astroid@bobo.none>
Date:   Sun, 28 Nov 2021 21:08:41 +1000
From:   Nicholas Piggin <npiggin@...il.com>
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Subject: Re: [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage

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