Re: [Patch v4 02/22] sched/cache: Limit the scan number of CPUs when calculating task occupancy

[Luo Gengkun]

On 2026/4/2 5:52, Tim Chen wrote:
> From: Chen Yu <yu.c.chen@xxxxxxxxx>
>
> When NUMA balancing is enabled, the kernel currently iterates over all
> online CPUs to aggregate process-wide occupancy data. On large systems,
> this global scan introduces significant overhead.
>
> To reduce scan latency, limit the search to a subset of relevant CPUs:
> 1. The task's preferred NUMA node.
> 2. The node where the task is currently running.
> 3. The node that contains the task's current preferred LLC..
>
> While focusing solely on the preferred NUMA node is ideal, a
> process-wide scan must remain flexible because the "preferred node"
> is a per-task attribute. Different threads within the same process may
> have different preferred nodes, causing the process-wide preference to
> migrate. Maintaining a mask that covers both the preferred and active
> running nodes ensures accuracy while significantly reducing the number of
> CPUs inspected.

To address the issue of scanning overhead, there is a more targeted 
approach: only scanning the CPUs actually accessed by the process, and 
evicting these CPUs when they remain unaccessed for a specific period of 
time.

This significantly reduces unnecessary scanning in most scenario. I have 
attached the patch below for review. Please feel free to integrate or 
modify these changes.

Thansk!
Luo Gengkun

From de33b29f31a882ac8c0038cac4ec659c7fdfe60d Mon Sep 17 00:00:00 2001 

From: Luo Gengkun <luogengkun2@xxxxxxxxxx> 

Date: Thu, 9 Apr 2026 07:22:16 +0000 

Subject: [PATCH] sched/cache: Reduce the overhead of task_cache_work by 
only
 scan the visisted cpus. 



The overhead of task_cache_work is high, espeically in multi-NUMA 
system.
Currently, task_cache_work try to find the pref_llc by scan all cpus in 
the
system. However, most of these scans are meaningless, such as those for 

cpus that have never been visited or were accessed a long time ago. 



To address this problem, this patch introduces visited_cpus to track the 

visited cpus and uses llc_epoch_visited_timeout to evict cpus that have 

timed out. 



Signed-off-by: Luo Gengkun <luogengkun2@xxxxxxxxxx> 

--- 

 include/linux/sched.h |  1 + 

 kernel/sched/debug.c  |  2 ++ 

 kernel/sched/fair.c   | 19 ++++++++++++++++--- 

 kernel/sched/sched.h  |  1 + 

 4 files changed, 20 insertions(+), 3 deletions(-) 



diff --git a/include/linux/sched.h b/include/linux/sched.h 

index dfa4bfd099c6..f2327a13fda8 100644 

--- a/include/linux/sched.h 

+++ b/include/linux/sched.h 

@@ -2390,6 +2390,7 @@ struct sched_cache_time { 



 struct sched_cache_stat { 

        struct sched_cache_time __percpu *pcpu_sched; 

+       struct cpumask visited_cpus; 

        raw_spinlock_t lock; 

        unsigned long epoch; 

        u64 nr_running_avg; 

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c 

index 4469e1c152c8..da66acabff4f 100644 

--- a/kernel/sched/debug.c 

+++ b/kernel/sched/debug.c 

@@ -675,6 +675,8 @@ static __init int sched_init_debug(void) 

                           &llc_epoch_period); 

        debugfs_create_u32("epoch_affinity_timeout", 0644, llc, 

                           &llc_epoch_affinity_timeout); 

+       debugfs_create_u32("epoch_visited_timeout", 0644, llc, 

+                          &llc_epoch_visited_timeout); 

        debugfs_create_u32("overaggr_pct", 0644, llc, 

                           &llc_overaggr_pct); 

        debugfs_create_u32("imb_pct", 0644, llc, 

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c 

index e4e22696a0b1..4ccabc412aa1 100644 

--- a/kernel/sched/fair.c 

+++ b/kernel/sched/fair.c 

@@ -1285,6 +1285,7 @@ static void set_next_buddy(struct sched_entity 
*se);
 __read_mostly unsigned int llc_aggr_tolerance  = 1; 

 __read_mostly unsigned int llc_epoch_period    = EPOCH_PERIOD; 

 __read_mostly unsigned int llc_epoch_affinity_timeout = 
EPOCH_LLC_AFFINITY_TIMEOUT;
+__read_mostly unsigned int llc_epoch_visited_timeout = 
EPOCH_LLC_AFFINITY_TIMEOUT;
 __read_mostly unsigned int llc_imb_pct         = 20; 

 __read_mostly unsigned int llc_overaggr_pct    = 50; 



@@ -1466,6 +1467,7 @@ void mm_init_sched(struct mm_struct *mm, 

        raw_spin_lock_init(&mm->sc_stat.lock); 

        mm->sc_stat.epoch = epoch; 

        mm->sc_stat.cpu = -1; 

+       cpumask_clear(&mm->sc_stat.visited_cpus); 



        /* 

         * The update to mm->sc_stat should not be reordered 

@@ -1582,6 +1584,7 @@ void account_mm_sched(struct rq *rq, struct 
task_struct *p, s64 delta_exec)
                pcpu_sched->runtime += delta_exec; 

                rq->cpu_runtime += delta_exec; 

                epoch = rq->cpu_epoch; 

+               cpumask_set_cpu(cpu_of(rq), &mm->sc_stat.visited_cpus); 

        } 



        /* 

@@ -1724,7 +1727,10 @@ static void task_cache_work(struct callback_head 
*work)
                return; 



        scoped_guard (cpus_read_lock) { 

-               get_scan_cpumasks(cpus, p); 

+               if (unlikely(llc_epoch_visited_timeout == 0)) 

+                       get_scan_cpumasks(cpus, p); 

+               else 

+                       cpumask_and(cpus, cpu_online_mask, 
&mm->sc_stat.visited_cpus);


                for_each_cpu(cpu, cpus) { 

                        /* XXX sched_cluster_active */ 

@@ -1736,8 +1742,15 @@ static void task_cache_work(struct callback_head 
*work)
                                continue; 



                        for_each_cpu(i, sched_domain_span(sd)) { 

-                               occ = fraction_mm_sched(cpu_rq(i), 

- 
per_cpu_ptr(mm->sc_stat.pcpu_sched, i));
+                               struct rq *rq = cpu_rq(i); 

+                               struct sched_cache_time *pcpu_sched = 
per_cpu_ptr(mm->sc_stat.pcpu_sched, i);
+                               /* Skip the rq that has not been hit for 
a long time */
+                               if (llc_epoch_visited_timeout && 
(rq->cpu_epoch - pcpu_sched->epoch) >
+                                   llc_epoch_visited_timeout) { 

+                                       cpumask_clear_cpu(cpu_of(rq), 
&mm->sc_stat.visited_cpus);
+                                       continue; 

+                               }
+                               occ = fraction_mm_sched(rq, pcpu_sched);
                                a_occ += occ;
                                if (occ > m_occ) {
                                        m_occ = occ;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b757812725f7..93908bb17cc5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -4041,6 +4041,7 @@ extern int sysctl_sched_cache_user;
 extern unsigned int llc_aggr_tolerance;
 extern unsigned int llc_epoch_period;
 extern unsigned int llc_epoch_affinity_timeout;
+extern unsigned int llc_epoch_visited_timeout;
 extern unsigned int llc_imb_pct;
 extern unsigned int llc_overaggr_pct;

--
2.34.1 


> Future work may integrate numa_group to further refine task aggregation.
>
> Suggested-by: Madadi Vineeth Reddy <vineethr@xxxxxxxxxxxxx>
> Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> Co-developed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
>
> Notes:
>      v3->v4:
>         New patch.
>
>   kernel/sched/fair.c | 46 ++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 45 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index eb3cfb852a93..20a33900f4ea 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1431,6 +1431,50 @@ static void task_tick_cache(struct rq *rq, struct task_struct *p)
>   	}
>   }
>   
> +static void get_scan_cpumasks(cpumask_var_t cpus, struct task_struct *p)
> +{
> +#ifdef CONFIG_NUMA_BALANCING
> +	int cpu, curr_cpu, pref_nid;
> +
> +	if (!static_branch_likely(&sched_numa_balancing))
> +		goto out;
> +
> +	cpu = p->mm->sc_stat.cpu;
> +	curr_cpu = task_cpu(p);
> +
> +	/*
> +	 * Scanning in the preferred NUMA node is ideal. However, the NUMA
> +	 * preferred node is per-task rather than per-process. It is possible
> +	 * for different threads of the process to have distinct preferred
> +	 * nodes; consequently, the process-wide preferred LLC may bounce
> +	 * between different nodes. As a workaround, maintain the scan
> +	 * CPU mask to also cover the process's current preferred LLC and the
> +	 * current running node to mitigate the bouncing risk.
> +	 * TBD: numa_group should be considered during task aggregation.
> +	 */
> +	pref_nid = p->numa_preferred_nid;
> +	/* honor the task's preferred node */
> +	if (pref_nid == NUMA_NO_NODE)
> +		goto out;
> +
> +	cpumask_or(cpus, cpus, cpumask_of_node(pref_nid));
> +
> +	/* honor the task's preferred LLC CPU */
> +	if (cpu != -1 && !cpumask_test_cpu(cpu, cpus))
> +		cpumask_or(cpus, cpus,
> +			   cpumask_of_node(cpu_to_node(cpu)));
> +
> +	/* make sure the task's current running node is included */
> +	if (!cpumask_test_cpu(curr_cpu, cpus))
> +		cpumask_or(cpus, cpus, cpumask_of_node(cpu_to_node(curr_cpu)));
> +
> +	return;
> +
> +out:
> +#endif
> +	cpumask_copy(cpus, cpu_online_mask);
> +}
> +
>   static void task_cache_work(struct callback_head *work)
>   {
>   	struct task_struct *p = current;
> @@ -1451,7 +1495,7 @@ static void task_cache_work(struct callback_head *work)
>   		return;
>   
>   	scoped_guard (cpus_read_lock) {
> -		cpumask_copy(cpus, cpu_online_mask);
> +		get_scan_cpumasks(cpus, p);
>   
>   		for_each_cpu(cpu, cpus) {
>   			/* XXX sched_cluster_active */