From mboxrd@z Thu Jan  1 00:00:00 1970
From: Yury Norov <yury.norov@gmail.com>
Subject: Re: [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage
Date: Sun, 28 Nov 2021 15:36:27 -0800
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References: <20211128035704.270739-1-yury.norov@gmail.com>
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To: Nicholas Piggin <npiggin@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>, Andy Gross <agross@kernel.org>, David Airlie <airlied@linux.ie>, Alexey Klimov <aklimov@redhat.com>, Andi Kleen <ak@linux.intel.com>, Andrew Morton <akpm@linux-foundation.org>, Alexander Shishkin <alexander.shishkin@linux.intel.com>, Amitkumar Karwar <amitkarwar@gmail.com>, Andrew Lunn <andrew@lunn.ch>, Andy Shevchenko <andy@infradead.org>, Anup Patel <anup.patel@wdc.com>, Ard Biesheuvel <ardb@kernel.org>, Arnd Bergmann <arnd@arndb.de>, Jens Axboe <axboe@fb.com>, bcm-kernel-feedback-list@broadcom.com, Borislav Petkov <bp@alien8.de>, Catalin Marinas <catalin.marinas@arm.com>, Christoph Lameter <cl@linux.com>, Daniel Vetter <daniel@ffwll.ch>, Dave Hansen <dave.hansen@linux.intel.com>, David Laight <David.Laight@ACULAB.COM>, Dennis Zhou <dennis@kernel.org>, Dinh Nguyen <dinguyen@kernel.org>, Geetha sowjanya <gakula@marvell.com>

On Sun, Nov 28, 2021 at 09:08:41PM +1000, Nicholas Piggin wrote:
> Excerpts from Yury Norov's message of November 28, 2021 1:56 pm:
> > In many cases people use bitmap_weight()-based functions like this:
> > 
> > 	if (num_present_cpus() > 1)
> > 		do_something();
> > 
> > This may take considerable amount of time on many-cpus machines because
> > num_present_cpus() 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 present cpus to any number greater than 1:
> > 
> > 	if (num_present_cpus_gt(1))
> > 		do_something();
> > 
> > To implement this idea, the series adds bitmap_weight_{eq,gt,le}
> > functions together with corresponding wrappers in cpumask and nodemask.
> 
> There would be no change to callers if you maintain counters like what
> is done for num_online_cpus() today. Maybe some fixes to arch code that
> does not use set_cpu_possible() etc APIs required, but AFAIKS it would
> be better to fix such cases anyway.

Thanks, Nick. I'll try to do this.