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Message-ID: <bea56432-09ec-496f-a95d-c04a779bbcf2@gmail.com>
Date: Wed, 17 Dec 2025 18:04:06 +0800
From: Vern Hao <haoxing990@...il.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
K Prateek Nayak <kprateek.nayak@....com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
Vincent Guittot <vincent.guittot@...aro.org>
Cc: Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
Hillf Danton <hdanton@...a.com>, Shrikanth Hegde <sshegde@...ux.ibm.com>,
Jianyong Wu <jianyong.wu@...look.com>, Yangyu Chen <cyy@...self.name>,
Tingyin Duan <tingyin.duan@...il.com>, Vern Hao <vernhao@...cent.com>,
Len Brown <len.brown@...el.com>, Aubrey Li <aubrey.li@...el.com>,
Zhao Liu <zhao1.liu@...el.com>, Chen Yu <yu.chen.surf@...il.com>,
Chen Yu <yu.c.chen@...el.com>, Adam Li <adamli@...amperecomputing.com>,
Aaron Lu <ziqianlu@...edance.com>, Tim Chen <tim.c.chen@...el.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 06/23] sched/cache: Track LLC-preferred tasks per
runqueue
On 2025/12/4 07:07, Tim Chen wrote:
> For each runqueue, track the number of tasks with an LLC preference
> and how many of them are running on their preferred LLC. This mirrors
> nr_numa_running and nr_preferred_running for NUMA balancing, and will
> be used by cache-aware load balancing in later patches.
>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>
> Notes:
> v1->v2: Invoke task_of() once and reuse its result afterwards.
> (Peter Zijlstra)
> Remove hacky reset_llc_stats() and introduce sched_llc_active flag
> to properly pair enqueue/dequeue statistics update (Peter Zijlstra, K Prateek Nayak)
>
> include/linux/sched.h | 2 ++
> init/init_task.c | 1 +
> kernel/sched/core.c | 5 ++++
> kernel/sched/fair.c | 60 ++++++++++++++++++++++++++++++++++++++++---
> kernel/sched/sched.h | 6 +++++
> 5 files changed, 71 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 1ad46220cd04..466ba8b7398c 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1408,6 +1408,8 @@ struct task_struct {
>
> #ifdef CONFIG_SCHED_CACHE
> struct callback_head cache_work;
> + /*the p is currently refcounted in a rq's preferred llc stats*/
> + bool sched_llc_active;
> int preferred_llc;
> #endif
>
> diff --git a/init/init_task.c b/init/init_task.c
> index 44bae72b5b7d..ee78837b0aa2 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -192,6 +192,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = {
> .numa_faults = NULL,
> #endif
> #ifdef CONFIG_SCHED_CACHE
> + .sched_llc_active = false,
> .preferred_llc = -1,
> #endif
> #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e8bdf03a4b7f..48626c81ba8e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -531,6 +531,11 @@ void __trace_set_current_state(int state_value)
> }
> EXPORT_SYMBOL(__trace_set_current_state);
>
> +int task_llc(const struct task_struct *p)
> +{
> + return per_cpu(sd_llc_id, task_cpu(p));
> +}
> +
> /*
> * Serialization rules:
> *
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 10cec83f65d5..d46a70a9d9fb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1223,6 +1223,43 @@ static int llc_id(int cpu)
> return llc;
> }
>
> +static void account_llc_enqueue(struct rq *rq, struct task_struct *p)
> +{
> + int pref_llc;
> +
> + if (!sched_cache_enabled())
> + return;
> +
> + pref_llc = p->preferred_llc;
> + if (pref_llc < 0)
> + return;
> +
> + rq->nr_llc_running++;
> + rq->nr_pref_llc_running += (pref_llc == task_llc(p));
> + p->sched_llc_active = true;
> +}
> +
> +static void account_llc_dequeue(struct rq *rq, struct task_struct *p)
> +{
> + int pref_llc;
> +
> + /*
> + * Borrow the uc_se->active from uclamp_rq_inc_id(),
> + * uclamp_rq_dec_id() to avoid the unbalanced calculation
> + * of rq statistics.
> + */
> + if (unlikely(!p->sched_llc_active))
> + return;
> +
> + pref_llc = p->preferred_llc;
> + if (pref_llc < 0)
> + return;
> +
> + rq->nr_llc_running--;
> + rq->nr_pref_llc_running -= (pref_llc == task_llc(p));
> + p->sched_llc_active = false;
> +}
> +
> void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)
> {
> unsigned long epoch;
> @@ -1294,6 +1331,8 @@ static unsigned long __no_profile fraction_mm_sched(struct rq *rq, struct mm_sch
> return div64_u64(NICE_0_LOAD * pcpu_sched->runtime, rq->cpu_runtime + 1);
> }
>
> +static unsigned int task_running_on_cpu(int cpu, struct task_struct *p);
> +
> static inline
> void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
> {
> @@ -1346,8 +1385,13 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
> #endif
> }
>
> - if (p->preferred_llc != mm_sched_llc)
> + /* task not on rq accounted later in account_entity_enqueue() */
> + if (task_running_on_cpu(rq->cpu, p) &&
> + p->preferred_llc != mm_sched_llc) {
#ifdef CONFIG_NUMA_BALANCING
/*
* Don't assign preferred LLC if it
* conflicts with NUMA balancing.
*/
if (p->numa_preferred_nid >= 0 &&
cpu_to_node(mm->mm_sched_cpu) != p->numa_preferred_nid)
mm_sched_llc = -1;
#endif
}
/* task not on rq accounted later in account_entity_enqueue() */
if (task_running_on_cpu(rq->cpu, p) &&
p->preferred_llc != mm_sched_llc) {
account_llc_dequeue(rq, p);
p->preferred_llc = mm_sched_llc;
account_llc_enqueue(rq, p);
}
I am a little concerned that there might be cases where both
|p->preferred_llc| and |mm_sched_llc| are equal to -1 at this point.",
Is it necessary to add a check here?
> + account_llc_dequeue(rq, p);
> p->preferred_llc = mm_sched_llc;
> + account_llc_enqueue(rq, p);
> + }
> }
>
> static void task_tick_cache(struct rq *rq, struct task_struct *p)
> @@ -1475,6 +1519,10 @@ void init_sched_mm(struct task_struct *p) { }
>
> static void task_tick_cache(struct rq *rq, struct task_struct *p) { }
>
> +static void account_llc_enqueue(struct rq *rq, struct task_struct *p) {}
> +
> +static void account_llc_dequeue(struct rq *rq, struct task_struct *p) {}
> +
> #endif
>
> /*
> @@ -3965,9 +4013,11 @@ account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> update_load_add(&cfs_rq->load, se->load.weight);
> if (entity_is_task(se)) {
> + struct task_struct *p = task_of(se);
> struct rq *rq = rq_of(cfs_rq);
>
> - account_numa_enqueue(rq, task_of(se));
> + account_numa_enqueue(rq, p);
> + account_llc_enqueue(rq, p);
> list_add(&se->group_node, &rq->cfs_tasks);
> }
> cfs_rq->nr_queued++;
> @@ -3978,7 +4028,11 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> update_load_sub(&cfs_rq->load, se->load.weight);
> if (entity_is_task(se)) {
> - account_numa_dequeue(rq_of(cfs_rq), task_of(se));
> + struct task_struct *p = task_of(se);
> + struct rq *rq = rq_of(cfs_rq);
> +
> + account_numa_dequeue(rq, p);
> + account_llc_dequeue(rq, p);
> list_del_init(&se->group_node);
> }
> cfs_rq->nr_queued--;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 728737641847..ee8b70647835 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1126,6 +1126,10 @@ struct rq {
> unsigned int nr_preferred_running;
> unsigned int numa_migrate_on;
> #endif
> +#ifdef CONFIG_SCHED_CACHE
> + unsigned int nr_pref_llc_running;
> + unsigned int nr_llc_running;
> +#endif
> #ifdef CONFIG_NO_HZ_COMMON
> unsigned long last_blocked_load_update_tick;
> unsigned int has_blocked_load;
> @@ -1980,6 +1984,8 @@ init_numa_balancing(u64 clone_flags, struct task_struct *p)
>
> #endif /* !CONFIG_NUMA_BALANCING */
>
> +int task_llc(const struct task_struct *p);
> +
> static inline void
> queue_balance_callback(struct rq *rq,
> struct balance_callback *head,
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