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

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Message-ID: <d7dfe129-157f-f7ae-0fdb-f5eafef1f627@fi.rohmeurope.com>
Date:   Wed, 26 Jan 2022 07:30:03 +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 v3 00/54] lib/bitmap: optimize bitmap_weight() usage

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()?

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.

Best Regards
	-- Matti Vaittinen


-- 
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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