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[73.224.175.84]) by smtp.gmail.com with ESMTPSA id 3f1490d57ef6-e9342de8fc9sm3158696276.18.2025.08.19.07.14.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Aug 2025 07:14:32 -0700 (PDT) Date: Tue, 19 Aug 2025 10:14:31 -0400 From: Yury Norov To: Valentin Schneider Cc: Srikar Dronamraju , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: simplify sched_init_numa() Message-ID: References: <20250719210753.401129-1-yury.norov@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Aug 19, 2025 at 10:52:19AM +0200, Valentin Schneider wrote: > On 21/07/25 14:02, Srikar Dronamraju wrote: > > * Yury Norov [2025-07-19 17:07:51]: > > > >> From: Yury Norov (NVIDIA) > >> > >> The function opencodes for_each_set_bit() macro. > >> > >> Signed-off-by: Yury Norov (NVIDIA) > >> --- > >> kernel/sched/topology.c | 7 +++---- > >> 1 file changed, 3 insertions(+), 4 deletions(-) > >> > >> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c > >> index b958fe48e020..7dc3c79aa480 100644 > >> --- a/kernel/sched/topology.c > >> +++ b/kernel/sched/topology.c > >> @@ -1932,10 +1932,9 @@ void sched_init_numa(int offline_node) > >> return; > >> } > >> > >> - for (i = 0, j = 0; i < nr_levels; i++, j++) { > >> - j = find_next_bit(distance_map, NR_DISTANCE_VALUES, j); > >> - distances[i] = j; > >> - } > >> + for_each_set_bit(j, distance_map, NR_DISTANCE_VALUES) > >> + distances[i++] = j; > >> + > > > > Dont we need to reset the value of i; > > > > That. Yes we need. > > Also now we may be iterating for NR_DISTANCE_VALUES instead of nr_levels. > > It should be okay, since NR_DISTANCE_VALUES is just 8. > > > > DISTANCE_BITS is 8, NR_DISTANCE_VALUES is 1 << 8 aka 256. I mistaken this DISTANCE_BITS vs NR_DISTANCE_VALUES. The first one is 8, so we can benefit from small_const_nbits() optimization. But we actually iterate over NR_DISTANCE_VALUES - and no benefit for using it over nr_levels. > But here the use of for_each_set_bit() means we'll get just one extra > iteration compared to using `nr_levels`. That said, since we *have* to > compute `nr_levels` to allocate sched_domains_numa_distance, IMO we're not > gaining much by using for_each_set_bit() here. I'm not sure I understand that. If we switch the loop terminator to nr_levels, the for_each() will be the exact functional equivalent. I'll get back to this function shortly and send v2. Thanks for the discussion.