linux-kernel - Re: [PATCH v3 00/54] lib/bitmap: optimize bitmap

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Date:   Thu, 27 Jan 2022 09:44:21 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     "Vaittinen, Matti" <matti.vaittinen@...rohmeurope.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Joe Perches <joe@...ches.com>, Dennis Zhou <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Nicholas Piggin <npiggin@...il.com>,
        Alexey Klimov <aklimov@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 00/54] lib/bitmap: optimize bitmap_weight() usage

On Tue, Jan 25, 2022 at 11:30 PM Vaittinen, Matti
<Matti.Vaittinen@...rohmeurope.com> wrote:
>
> On 1/23/22 20:38, Yury Norov wrote:
> > In many cases people use bitmap_weight()-based functions to compare
> > the result against a number of expression:
> >
> >       if (cpumask_weight(mask) > 1)
> >               do_something();
> >
> > This may take considerable amount of time on many-cpus machines because
> > cpumask_weight() 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 cpus to any number greater than 1:
> >
> >       if (cpumask_weight_gt(mask, 1))
> >               do_something();
>
> I guess I am part of the recipient list because I did the original
> suggestion of adding the single_bit_set()?

Yes, because of single_bit_set()

> If this is the case - well, I do like this series. Overall it looks good
> to me - but I for sure did not go through all the changes in detail ;)
> If there is some other reason to loop me in (Eg, if someone expects me
> to take a more specific look on something) - please give me a nudge.

The key patch of the series is #27: "lib/bitmap: add bitmap_weight_{cmp, eq,
gt, ge, lt, le} functions"

Feel free to add suggested/reviewed (or whatever you find appropriate) tags
if you want.

Thanks,
Yury