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Message-ID: <Y26BQ92l9xWKaz2z@yury-laptop> Date: Fri, 11 Nov 2022 09:07:15 -0800 From: Yury Norov <yury.norov@...il.com> To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com> Cc: linux-kernel@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, Barry Song <baohua@...nel.org>, Ben Segall <bsegall@...gle.com>, Daniel Bristot de Oliveira <bristot@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, Gal Pressman <gal@...dia.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Heiko Carstens <hca@...ux.ibm.com>, Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>, Jason Gunthorpe <jgg@...dia.com>, Jesse Brandeburg <jesse.brandeburg@...el.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, Juri Lelli <juri.lelli@...hat.com>, Leon Romanovsky <leonro@...dia.com>, Mel Gorman <mgorman@...e.de>, Peter Zijlstra <peterz@...radead.org>, Rasmus Villemoes <linux@...musvillemoes.dk>, Saeed Mahameed <saeedm@...dia.com>, Steven Rostedt <rostedt@...dmis.org>, Tariq Toukan <tariqt@...dia.com>, Tariq Toukan <ttoukan.linux@...il.com>, Tony Luck <tony.luck@...el.com>, Valentin Schneider <vschneid@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, linux-crypto@...r.kernel.org, netdev@...r.kernel.org, linux-rdma@...r.kernel.org Subject: Re: [PATCH 3/4] sched: add sched_numa_find_nth_cpu() On Fri, Nov 11, 2022 at 01:42:29PM +0200, Andy Shevchenko wrote: > On Thu, Nov 10, 2022 at 08:00:26PM -0800, Yury Norov wrote: > > The function finds Nth set CPU in a given cpumask starting from a given > > node. > > > > Leveraging the fact that each hop in sched_domains_numa_masks includes the > > same or greater number of CPUs than the previous one, we can use binary > > search on hops instead of linear walk, which makes the overall complexity > > of O(log n) in terms of number of cpumask_weight() calls. > > ... > > > +int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node) > > +{ > > + unsigned int first = 0, mid, last = sched_domains_numa_levels; > > + struct cpumask ***masks; > > *** ? > Hmm... Do we really need such deep indirection? It's 2d array of pointers, so - yes. > > + int w, ret = nr_cpu_ids; > > + > > + rcu_read_lock(); > > + masks = rcu_dereference(sched_domains_numa_masks); > > + if (!masks) > > + goto out; > > + > > + while (last >= first) { > > + mid = (last + first) / 2; > > + > > + if (cpumask_weight_and(cpus, masks[mid][node]) <= cpu) { > > + first = mid + 1; > > + continue; > > + } > > + > > + w = (mid == 0) ? 0 : cpumask_weight_and(cpus, masks[mid - 1][node]); > > See below. > > > + if (w <= cpu) > > + break; > > + > > + last = mid - 1; > > + } > > We have lib/bsearch.h. I haven't really looked deeply into the above, but my > gut feelings that that might be useful here. Can you check that? Yes we do. I tried it, and it didn't work because nodes arrays are allocated dynamically, and distance between different pairs of hops for a given node is not a constant, which is a requirement for bsearch. However, distance between hops pointers in 1st level array should be constant, and we can try feeding bsearch with it. I'll experiment with bsearch for more. > > + ret = (mid == 0) ? > > + cpumask_nth_and(cpu - w, cpus, masks[mid][node]) : > > + cpumask_nth_and_andnot(cpu - w, cpus, masks[mid][node], masks[mid - 1][node]); > > You can also shorten this by inversing the conditional: > > ret = mid ? ...not 0... : ...for 0...; Yep, why not. > > +out: > > out_unlock: ? Do you think it's better? > > + rcu_read_unlock(); > > + return ret; > > +} > > -- > With Best Regards, > Andy Shevchenko >
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