linux-kernel - Re: [PATCH 27/54] lib/bitmap: add bitmap_weight

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Date:   Fri, 28 Jan 2022 06:59:14 +0000
From:   "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>
To:     Yury Norov <yury.norov@...il.com>,
        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 27/54] lib/bitmap: add bitmap_weight_{cmp, eq, gt, ge, lt,
 le} functions

On 1/23/22 20:38, Yury Norov wrote:
> Many kernel users use bitmap_weight() to compare the result against
> some number or expression:
> 
> 	if (bitmap_weight(...) > 1)
> 		do_something();
> 
> It works OK, but may be significantly improved for large bitmaps: if
> first few words count set bits to a number greater than given, we can
> stop counting and immediately return.
> 
> The same idea would work in other direction: if we know that the number
> of set bits that we counted so far is small enough, so that it would be
> smaller than required number even if all bits of the rest of the bitmap
> are set, we can stop counting earlier.
> 
> This patch adds new bitmap_weight_cmp() as suggested by Michał Mirosław
> and a family of eq, gt, ge, lt and le wrappers to allow this optimization.
> The following patches apply new functions where appropriate.
> 

Thanks for pushing this improvement Yury. Seeing how much this has 
evolved from the single_bit_set() suggestion - it'd be a bit thick from 
me to add a suggested-by ;) I did review it though and it looks good to me!

Reviewed-by: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>

> Suggested-by: "Michał Mirosław" <mirq-linux@...e.qmqm.pl> (for bitmap_weight_cmp)
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>   include/linux/bitmap.h | 80 ++++++++++++++++++++++++++++++++++++++++++
>   lib/bitmap.c           | 21 +++++++++++
>   2 files changed, 101 insertions(+)


-- 
The Linux Kernel guy at ROHM Semiconductors

Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

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