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

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Date:   Sun, 28 Nov 2021 15:36:27 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
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Subject: 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:
> > 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.