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Mon, 26 Jan 2026 03:02:59 -0800 (PST) From: Qiliang Yuan To: kprateek.nayak@amd.com Cc: bsegall@google.com, dietmar.eggemann@arm.com, juri.lelli@redhat.com, linux-kernel@vger.kernel.org, mgorman@suse.de, mingo@redhat.com, peterz@infradead.org, realwujing@gmail.com, rostedt@goodmis.org, vincent.guittot@linaro.org, vschneid@redhat.com, yuanql9@chinatelecom.cn Subject: [PATCH v3] sched/fair: Cache NUMA node statistics to avoid O(N) scanning Date: Mon, 26 Jan 2026 06:02:50 -0500 Message-ID: <20260126110250.1060512-1-realwujing@gmail.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: References: 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" Optimize update_numa_stats() by leveraging pre-calculated node statistics cached during the load balancing process. This reduces the complexity of NUMA balancing overhead from O(CPUs_per_node) to O(1) when statistics for the source node are fresh. Signed-off-by: Qiliang Yuan Signed-off-by: Qiliang Yuan --- kernel/sched/fair.c | 44 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index e71302282671..070b61f65b6d 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2094,6 +2094,17 @@ static inline int numa_idle_core(int idle_core, int = cpu) * borrows code and logic from update_sg_lb_stats but sharing a * common implementation is impractical. */ +struct numa_stats_cache { + unsigned long load; + unsigned long runnable; + unsigned long util; + unsigned long nr_running; + unsigned long capacity; + unsigned long last_update; +}; + +static struct numa_stats_cache node_stats_cache[MAX_NUMNODES]; + static void update_numa_stats(struct task_numa_env *env, struct numa_stats *ns, int nid, bool find_idle) @@ -2104,6 +2115,24 @@ static void update_numa_stats(struct task_numa_env *= env, ns->idle_cpu =3D -1; =20 rcu_read_lock(); + /* + * Algorithmic Optimization: Avoid O(N) scan by using cached stats. + * Only applicable for the source node where we don't need to find + * an idle CPU. + */ + if (!find_idle && nid =3D=3D env->src_nid) { + struct numa_stats_cache *cache =3D &node_stats_cache[nid]; + + if (time_before(jiffies, cache->last_update + msecs_to_jiffies(10))) { + ns->load =3D READ_ONCE(cache->load); + ns->runnable =3D READ_ONCE(cache->runnable); + ns->util =3D READ_ONCE(cache->util); + ns->nr_running =3D READ_ONCE(cache->nr_running); + ns->compute_capacity =3D READ_ONCE(cache->capacity); + goto skip_scan; + } + } + for_each_cpu(cpu, cpumask_of_node(nid)) { struct rq *rq =3D cpu_rq(cpu); =20 @@ -2124,6 +2153,8 @@ static void update_numa_stats(struct task_numa_env *e= nv, idle_core =3D numa_idle_core(idle_core, cpu); } } + +skip_scan: rcu_read_unlock(); =20 ns->weight =3D cpumask_weight(cpumask_of_node(nid)); @@ -10488,6 +10519,19 @@ static inline void update_sg_lb_stats(struct lb_en= v *env, if (sgs->group_type =3D=3D group_overloaded) sgs->avg_load =3D (sgs->group_load * SCHED_CAPACITY_SCALE) / sgs->group_capacity; + + /* Algorithmic Optimization: Cache node stats for O(1) NUMA lookups */ + if (env->sd->flags & SD_NUMA) { + int nid =3D cpu_to_node(cpumask_first(sched_group_span(group))); + struct numa_stats_cache *cache =3D &node_stats_cache[nid]; + + WRITE_ONCE(cache->nr_running, sgs->sum_h_nr_running); + WRITE_ONCE(cache->load, sgs->group_load); + WRITE_ONCE(cache->util, sgs->group_util); + WRITE_ONCE(cache->runnable, sgs->group_runnable); + WRITE_ONCE(cache->capacity, sgs->group_capacity); + WRITE_ONCE(cache->last_update, jiffies); + } } =20 /** --=20 2.51.0