From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yury Norov Date: Mon, 29 Nov 2021 06:38:39 +0000 Subject: Re: [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage Message-Id: <20211129063839.GA338729@lapt> List-Id: References: <20211128035704.270739-1-yury.norov@gmail.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: mirq-test@rere.qmqm.pl Cc: linux-kernel@vger.kernel.org, Yury Norov , "James E.J. Bottomley" , "Martin K. Petersen" , "Paul E. McKenney" , "Rafael J. Wysocki" , Alexander Shishkin , Alexey Klimov , Amitkumar Karwar , Andi Kleen , Andrew Lunn , Andrew Morton , Andy Gross , Andy Lutomirski , Andy Shevchenko , Anup Patel , Ard Biesheuvel , Arnaldo Carvalho de Melo , Arnd Bergmann , Borislav Petkov , Catalin Marinas , Christoph Hellwig , Christoph Lameter , Daniel Vetter , Dave Hansen , David Airlie , David Laight , Dennis Zhou , Dinh Nguyen , Geetha sowjanya , Geert Uytterhoeven , Greg Kroah-Hartman , Guo Ren , Hans de Goede , Heiko Carstens , Ian Rogers , Ingo Molnar , Jakub Kicinski , Jason Wessel , Jens Axboe , Jiri Olsa , Jonathan Cameron , Juri Lelli , Kalle Valo , Kees Cook , Krzysztof Kozlowski , Lee Jones , Marc Zyngier , Marcin Wojtas , Mark Gross , Mark Rutland , Matti Vaittinen , Mauro Carvalho Chehab , Mel Gorman , Michael Ellerman , Mike Marciniszyn , Nicholas Piggin , Palmer Dabbelt , Peter Zijlstra , Petr Mladek , Randy Dunlap , Rasmus Villemoes , Roy Pledge , Russell King , Saeed Mahameed , Sagi Grimberg , Sergey Senozhatsky , Solomon Peachy , Stephen Boyd , Stephen Rothwell , Steven Rostedt , Subbaraya Sundeep , Sudeep Holla , Sunil Goutham , Tariq Toukan , Tejun Heo , Thomas Bogendoerfer , Thomas Gleixner , Ulf Hansson , Vincent Guittot , Vineet Gupta , Viresh Kumar , Vivien Didelot , Vlastimil Babka , Will Deacon , bcm-kernel-feedback-list@broadcom.com, kvm@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-csky@vger.kernel.org, linux-ia64@vger.kernel.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, linux-perf-users@vger.kernel.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-snps-arc@lists.infradead.org, linuxppc-dev@lists.ozlabs.org On Sun, Nov 28, 2021 at 07:03:41PM +0100, mirq-test@rere.qmqm.pl wrote: > On Sat, Nov 27, 2021 at 07:56:55PM -0800, Yury Norov wrote: > > 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. > > Having slept on it I have more structured thoughts: > > First, I like substituting bitmap_empty/full where possible - I think > the change stands on its own, so could be split and sent as is. Ok, I can do it. > I don't like the proposed API very much. One problem is that it hides > the comparison operator and makes call sites less readable: > > bitmap_weight(...) > N > > becomes: > > bitmap_weight_gt(..., N) > > and: > bitmap_weight(...) <= N > > becomes: > > bitmap_weight_lt(..., N+1) > or: > !bitmap_weight_gt(..., N) > > I'd rather see something resembling memcmp() API that's known enough > to be easier to grasp. For above examples: > > bitmap_weight_cmp(..., N) > 0 > bitmap_weight_cmp(..., N) <= 0 > ... bitmap_weight_cmp() cannot be efficient. Consider this example: bitmap_weight_lt(1000 0000 0000 0000, 1) = false ^ stop here bitmap_weight_cmp(1000 0000 0000 0000, 1) = 0 ^ stop here I agree that '_gt' is less verbose than '>', but the advantage of '_gt' over '>' is proportional to length of bitmap, and it means that this API should exist. > This would also make the implementation easier in not having to > copy and paste the code three times. Could also use a simple > optimization reducing code size: In the next version I'll reduce code duplication like this: bool bitmap_eq(..., N); bool bitmap_ge(..., N); #define bitmap_weight_gt(..., N) bitmap_weight_ge(..., N + 1) #define bitmap_weight_lt(..., N) !bitmap_weight_ge(..., N) #define bitmap_weight_le(..., N) !bitmap_weight_gt(..., N) Thanks, Yury