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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YaMPIXUYuP1Q5FrQ@qmqm.qmqm.pl>
Date:   Sun, 28 Nov 2021 06:09:53 +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 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. 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.

Hmm. This code already unnecessarily calls num_possible_cpus() multiple
times. Since it doesn't change after early init I would suggest just
calling it once here.

Best Regards
Michał Mirosław