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

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Message-ID: <YaMME60Jfiz5BeJF@qmqm.qmqm.pl>
Date:   Sun, 28 Nov 2021 05:56:51 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com, kvm@...r.kernel.org,
        linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-crypto@...r.kernel.org, linux-csky@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linux-perf-users@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 7/9] lib/cpumask: add
 num_{possible,present,active}_cpus_{eq,gt,le}

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. 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.

Best Regards
Michał Mirosław