linux-kernel - Re: [PATCH 7/9] lib/cpumask: add num_{possible,present,active}_cpus

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Message-ID: <20211128063447.GA270945@lapt>
Date:   Sat, 27 Nov 2021 22:34:47 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
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Subject: Re: [PATCH 7/9] lib/cpumask: add
 num_{possible,present,active}_cpus_{eq,gt,le}

(restore CC list)

On Sun, Nov 28, 2021 at 05:56:51AM +0100, Michał Mirosław wrote:
> On Sat, Nov 27, 2021 at 07:57:02PM -0800, Yury Norov wrote:
> > Add num_{possible,present,active}_cpus_{eq,gt,le} and replace num_*_cpus()
> > with one of new functions where appropriate. This allows num_*_cpus_*()
> > to return earlier depending on the condition.
> [...]
> > @@ -3193,7 +3193,7 @@ int __init pcpu_page_first_chunk(size_t reserved_size,
> >  
> >  	/* allocate pages */
> >  	j = 0;
> > -	for (unit = 0; unit < num_possible_cpus(); unit++) {
> > +	for (unit = 0; num_possible_cpus_gt(unit); unit++) {
> 
> This looks dubious.

Only this?

> The old version I could hope the compiler would call
> num_possible_cpus() only once if it's marked const or pure, but the
> alternative is going to count the bits every time making this a guaranteed
> O(n^2) even though the bitmap doesn't change.

num_possible_cpus() is not const neither pure. This is O(n^2) before and after.