* [PATCH] sched/cache: honor migrate_llc_task semantics in active load balance
@ 2026-08-01 12:17 Lu Wang
0 siblings, 0 replies; 2+ messages in thread
From: Lu Wang @ 2026-08-01 12:17 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, peterz, juri.lelli, vincent.guittot, dietmar.eggemann,
rostedt, bsegall, mgorman, vschneid, kprateek.nayak, Lu Wang
A passive load-balance pass marks group_llc_balance as migrate_llc_task
and queues active balance when it cannot move a task. The CPU stopper
callback constructs a fresh lb_env, so preserve the migration type on
the runqueue across the asynchronous boundary.
For CAS-directed active balance, reject a candidate whose preferred LLC
does not match the destination LLC. This keeps the fallback from moving
a task away from its preferred LLC.
Fixes: e4c9a4cb244a ("sched/cache: Add migrate_llc_task migration type for cache-aware balancing")
Signed-off-by: Lu Wang <wanglu.priv@gmail.com>
---
kernel/sched/fair.c | 25 ++++++++++++++++++++++---
kernel/sched/sched.h | 1 +
2 files changed, 23 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6..cf036a8a0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10645,6 +10645,18 @@ alb_break_llc(struct lb_env *env)
return false;
}
+/*
+ * Returns true if p's preferred LLC does not match the destination CPU,
+ * meaning this task should not be migrated under migrate_llc_task semantics.
+ */
+static inline bool
+migrate_llc_task_wrong_dst(struct task_struct *p, struct lb_env *env)
+{
+ return sched_cache_enabled() &&
+ env->migration_type == migrate_llc_task &&
+ READ_ONCE(p->preferred_llc) != llc_id(env->dst_cpu);
+}
+
/*
* Check if migrating task p from env->src_cpu to
* env->dst_cpu breaks LLC localiy.
@@ -10673,8 +10685,7 @@ static bool migrate_degrades_llc(struct task_struct *p, struct lb_env *env)
* run on env->dst_cpu, skip the tasks do not prefer
* env->dst_cpu, and find the one that prefers.
*/
- if (env->migration_type == migrate_llc_task &&
- READ_ONCE(p->preferred_llc) != llc_id(env->dst_cpu))
+ if (migrate_llc_task_wrong_dst(p, env))
return true;
if (can_migrate_llc_task(env->src_cpu,
@@ -10697,6 +10708,12 @@ alb_break_llc(struct lb_env *env)
return false;
}
+static inline bool
+migrate_llc_task_wrong_dst(struct task_struct *p, struct lb_env *env)
+{
+ return false;
+}
+
static inline bool
migrate_degrades_llc(struct task_struct *p, struct lb_env *env)
{
@@ -10796,7 +10813,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
* 4) too many balance attempts have failed.
*/
if (env->flags & LBF_ACTIVE_LB)
- return 1;
+ return !migrate_llc_task_wrong_dst(p, env);
degrades = migrate_degrades_locality(p, env);
if (!degrades) {
@@ -13485,6 +13502,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
if (!busiest->active_balance) {
busiest->active_balance = 1;
busiest->push_cpu = this_cpu;
+ busiest->active_balance_type = env.migration_type;
active_balance = 1;
}
@@ -13654,6 +13672,7 @@ static int active_load_balance_cpu_stop(void *data)
.src_rq = busiest_rq,
.idle = CPU_IDLE,
.flags = LBF_ACTIVE_LB,
+ .migration_type = (enum migration_type)busiest_rq->active_balance_type,
};
schedstat_inc(sd->alb_count);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 56acf502b..82084d405 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1266,6 +1266,7 @@ struct rq {
/* For active balancing */
int active_balance;
int push_cpu;
+ int active_balance_type; /* enum migration_type */
struct cpu_stop_work active_balance_work;
/* CPU of this runqueue: */
--
2.43.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* [PATCH] sched/cache: honor migrate_llc_task semantics in active load balance
@ 2026-08-01 12:22 Lu Wang
0 siblings, 0 replies; 2+ messages in thread
From: Lu Wang @ 2026-08-01 12:22 UTC (permalink / raw)
To: linux-kernel
Cc: tim.c.chen, yu.c.chen, peterz, mingo, juri.lelli, vincent.guittot,
dietmar.eggemann, rostedt, bsegall, mgorman, vschneid,
kprateek.nayak, Lu Wang
A passive load-balance pass marks group_llc_balance as migrate_llc_task
and queues active balance when it cannot move a task. The CPU stopper
callback constructs a fresh lb_env, so preserve the migration type on
the runqueue across the asynchronous boundary.
For CAS-directed active balance, reject a candidate whose preferred LLC
does not match the destination LLC. This keeps the fallback from moving
a task away from its preferred LLC.
Fixes: e4c9a4cb244a ("sched/cache: Add migrate_llc_task migration type for cache-aware balancing")
Signed-off-by: Lu Wang <wanglu.priv@gmail.com>
---
v2: add Tim Chen and Chen Yu to Cc
kernel/sched/fair.c | 25 ++++++++++++++++++++++---
kernel/sched/sched.h | 1 +
2 files changed, 23 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6..cf036a8a0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10645,6 +10645,18 @@ alb_break_llc(struct lb_env *env)
return false;
}
+/*
+ * Returns true if p's preferred LLC does not match the destination CPU,
+ * meaning this task should not be migrated under migrate_llc_task semantics.
+ */
+static inline bool
+migrate_llc_task_wrong_dst(struct task_struct *p, struct lb_env *env)
+{
+ return sched_cache_enabled() &&
+ env->migration_type == migrate_llc_task &&
+ READ_ONCE(p->preferred_llc) != llc_id(env->dst_cpu);
+}
+
/*
* Check if migrating task p from env->src_cpu to
* env->dst_cpu breaks LLC localiy.
@@ -10673,8 +10685,7 @@ static bool migrate_degrades_llc(struct task_struct *p, struct lb_env *env)
* run on env->dst_cpu, skip the tasks do not prefer
* env->dst_cpu, and find the one that prefers.
*/
- if (env->migration_type == migrate_llc_task &&
- READ_ONCE(p->preferred_llc) != llc_id(env->dst_cpu))
+ if (migrate_llc_task_wrong_dst(p, env))
return true;
if (can_migrate_llc_task(env->src_cpu,
@@ -10697,6 +10708,12 @@ alb_break_llc(struct lb_env *env)
return false;
}
+static inline bool
+migrate_llc_task_wrong_dst(struct task_struct *p, struct lb_env *env)
+{
+ return false;
+}
+
static inline bool
migrate_degrades_llc(struct task_struct *p, struct lb_env *env)
{
@@ -10796,7 +10813,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
* 4) too many balance attempts have failed.
*/
if (env->flags & LBF_ACTIVE_LB)
- return 1;
+ return !migrate_llc_task_wrong_dst(p, env);
degrades = migrate_degrades_locality(p, env);
if (!degrades) {
@@ -13485,6 +13502,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
if (!busiest->active_balance) {
busiest->active_balance = 1;
busiest->push_cpu = this_cpu;
+ busiest->active_balance_type = env.migration_type;
active_balance = 1;
}
@@ -13654,6 +13672,7 @@ static int active_load_balance_cpu_stop(void *data)
.src_rq = busiest_rq,
.idle = CPU_IDLE,
.flags = LBF_ACTIVE_LB,
+ .migration_type = (enum migration_type)busiest_rq->active_balance_type,
};
schedstat_inc(sd->alb_count);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 56acf502b..82084d405 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1266,6 +1266,7 @@ struct rq {
/* For active balancing */
int active_balance;
int push_cpu;
+ int active_balance_type; /* enum migration_type */
struct cpu_stop_work active_balance_work;
/* CPU of this runqueue: */
--
2.43.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
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