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Wed, 10 Sep 2025 14:08:57 -0700 (PDT) From: Yury Norov To: Thomas Gleixner , Yury Norov , Rasmus Villemoes , linux-kernel@vger.kernel.org Subject: [PATCH 2/3] group_cpus: don't call cpumask_weight() prematurely Date: Wed, 10 Sep 2025 17:08:47 -0400 Message-ID: <20250910210850.404834-3-yury.norov@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250910210850.404834-1-yury.norov@gmail.com> References: <20250910210850.404834-1-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-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: "Yury Norov (NVIDIA)" alloc_nodes_groups() and __group_cpus_evenly() call cpumask_weight() unconditionally in the for_each() loops. cpumask_weight() is O(N), so the complexity of the function becomes O(MAX_NUMNODES * nr_cpu_ids). This call may be avoided if the nmsk is empty. Signed-off-by: Yury Norov (NVIDIA) --- lib/group_cpus.c | 17 ++++++----------- 1 file changed, 6 insertions(+), 11 deletions(-) diff --git a/lib/group_cpus.c b/lib/group_cpus.c index 6d08ac05f371..6aae1560b796 100644 --- a/lib/group_cpus.c +++ b/lib/group_cpus.c @@ -142,15 +142,11 @@ static void alloc_nodes_groups(unsigned int numgrps, } =20 for_each_node_mask(n, nodemsk) { - unsigned ncpus; - - cpumask_and(nmsk, cpu_mask, node_to_cpumask[n]); - ncpus =3D cpumask_weight(nmsk); - - if (!ncpus) + if (!cpumask_and(nmsk, cpu_mask, node_to_cpumask[n])) continue; - remaining_ncpus +=3D ncpus; - node_groups[n].ncpus =3D ncpus; + + node_groups[n].ncpus =3D cpumask_weight(nmsk); + remaining_ncpus +=3D node_groups[n].ncpus; } =20 numgrps =3D min_t(unsigned, remaining_ncpus, numgrps); @@ -294,11 +290,10 @@ static int __group_cpus_evenly(unsigned int startgrp,= unsigned int numgrps, continue; =20 /* Get the cpus on this node which are in the mask */ - cpumask_and(nmsk, cpu_mask, node_to_cpumask[nv->id]); - ncpus =3D cpumask_weight(nmsk); - if (!ncpus) + if (!cpumask_and(nmsk, cpu_mask, node_to_cpumask[nv->id])) continue; =20 + ncpus =3D cpumask_weight(nmsk); WARN_ON_ONCE(nv->ngroups > ncpus); =20 /* Account for rounding errors */ --=20 2.43.0