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Message-ID: <20250702121159.652969404@infradead.org>
Date: Wed, 02 Jul 2025 13:49:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: mingo@...hat.com,
juri.lelli@...hat.com,
vincent.guittot@...aro.org,
dietmar.eggemann@....com,
rostedt@...dmis.org,
bsegall@...gle.com,
mgorman@...e.de,
vschneid@...hat.com,
clm@...a.com
Cc: linux-kernel@...r.kernel.org,
peterz@...radead.org
Subject: [PATCH v2 12/12] sched: Add ttwu_queue support for delayed tasks
One of the more expensive things to do is take a remote runqueue lock;
which is exactly what ttwu_runnable() ends up doing. However, in the
case of sched_delayed tasks it is possible to queue up an IPI instead.
Reported-by: Chris Mason <clm@...a.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20250520101727.984171377@infradead.org
---
include/linux/sched.h | 1
kernel/sched/core.c | 96 +++++++++++++++++++++++++++++++++++++++++++++---
kernel/sched/fair.c | 17 ++++++++
kernel/sched/features.h | 1
kernel/sched/sched.h | 1
5 files changed, 110 insertions(+), 6 deletions(-)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -984,6 +984,7 @@ struct task_struct {
* ->sched_remote_wakeup gets used, so it can be in this word.
*/
unsigned sched_remote_wakeup:1;
+ unsigned sched_remote_delayed:1;
#ifdef CONFIG_RT_MUTEXES
unsigned sched_rt_mutex:1;
#endif
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -675,7 +675,12 @@ struct rq *__task_rq_lock(struct task_st
{
struct rq *rq;
- lockdep_assert_held(&p->pi_lock);
+ /*
+ * TASK_WAKING is used to serialize the remote end of wakeup, rather
+ * than p->pi_lock.
+ */
+ lockdep_assert(p->__state == TASK_WAKING ||
+ lockdep_is_held(&p->pi_lock) != LOCK_STATE_NOT_HELD);
for (;;) {
rq = task_rq(p);
@@ -3727,6 +3732,8 @@ ttwu_do_activate(struct rq *rq, struct t
}
}
+static bool ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags);
+
/*
* Consider @p being inside a wait loop:
*
@@ -3754,6 +3761,35 @@ ttwu_do_activate(struct rq *rq, struct t
*/
static int ttwu_runnable(struct task_struct *p, int wake_flags)
{
+ if (sched_feat(TTWU_QUEUE_DELAYED) && READ_ONCE(p->se.sched_delayed)) {
+ /*
+ * Similar to try_to_block_task():
+ *
+ * __schedule() ttwu()
+ * prev_state = prev->state if (p->sched_delayed)
+ * if (prev_state) smp_acquire__after_ctrl_dep()
+ * try_to_block_task() p->state = TASK_WAKING
+ * ... set_delayed()
+ * RELEASE p->sched_delayed = 1
+ *
+ * __schedule() and ttwu() have matching control dependencies.
+ *
+ * Notably, once we observe sched_delayed we know the task has
+ * passed try_to_block_task() and p->state is ours to modify.
+ *
+ * TASK_WAKING controls ttwu() concurrency.
+ */
+ smp_acquire__after_ctrl_dep();
+ WRITE_ONCE(p->__state, TASK_WAKING);
+ /*
+ * Bit of a hack, see select_task_rq_fair()'s WF_DELAYED case.
+ */
+ p->wake_cpu = smp_processor_id();
+
+ if (ttwu_queue_wakelist(p, task_cpu(p), wake_flags | WF_DELAYED))
+ return 1;
+ }
+
CLASS(__task_rq_lock, guard)(p);
struct rq *rq = guard.rq;
@@ -3776,6 +3812,8 @@ static int ttwu_runnable(struct task_str
return 1;
}
+static void __ttwu_queue_wakelist(struct task_struct *p, int cpu);
+
static inline bool ttwu_do_migrate(struct rq *rq, struct task_struct *p, int cpu)
{
struct rq *p_rq = rq ? : task_rq(p);
@@ -3801,6 +3839,52 @@ static inline bool ttwu_do_migrate(struc
return true;
}
+static int ttwu_delayed(struct rq *rq, struct task_struct *p, int wake_flags,
+ struct rq_flags *rf)
+{
+ struct rq *p_rq = task_rq(p);
+ int cpu;
+
+ /*
+ * Notably it is possible for on-rq entities to get migrated -- even
+ * sched_delayed ones. This should be rare though, so flip the locks
+ * rather than IPI chase after it.
+ */
+ if (unlikely(rq != p_rq)) {
+ rq_unlock(rq, rf);
+ p_rq = __task_rq_lock(p, rf);
+ update_rq_clock(p_rq);
+ }
+
+ if (task_on_rq_queued(p))
+ dequeue_task(p_rq, p, DEQUEUE_NOCLOCK | DEQUEUE_SLEEP | DEQUEUE_DELAYED);
+
+ /*
+ * NOTE: unlike the regular try_to_wake_up() path, this runs both
+ * select_task_rq() and ttwu_do_migrate() while holding rq->lock
+ * rather than p->pi_lock.
+ */
+ cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
+ if (ttwu_do_migrate(rq, p, cpu))
+ wake_flags |= WF_MIGRATED;
+
+ if (unlikely(rq != p_rq)) {
+ __task_rq_unlock(p_rq, rf);
+ rq_lock(rq, rf);
+ }
+
+ p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED);
+ p->sched_remote_delayed = 0;
+
+ /* it wants to run here */
+ if (cpu_of(rq) == cpu)
+ return 0;
+
+ /* shoot it to the CPU it wants to run on */
+ __ttwu_queue_wakelist(p, cpu);
+ return 1;
+}
+
void sched_ttwu_pending(void *arg)
{
struct llist_node *llist = arg;
@@ -3819,12 +3903,13 @@ void sched_ttwu_pending(void *arg)
if (WARN_ON_ONCE(p->on_cpu))
smp_cond_load_acquire(&p->on_cpu, !VAL);
- if (WARN_ON_ONCE(task_cpu(p) != cpu_of(rq)))
- set_task_cpu(p, cpu_of(rq));
-
if (p->sched_remote_wakeup)
wake_flags |= WF_MIGRATED;
+ if (p->sched_remote_delayed &&
+ ttwu_delayed(rq, p, wake_flags | WF_DELAYED, &guard.rf))
+ continue;
+
ttwu_do_activate(rq, p, wake_flags, &guard.rf);
}
@@ -3964,12 +4049,13 @@ static inline bool ttwu_queue_cond(struc
static bool ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags)
{
- bool def = sched_feat(TTWU_QUEUE_DEFAULT);
+ bool def = sched_feat(TTWU_QUEUE_DEFAULT) || (wake_flags & WF_DELAYED);
if (!ttwu_queue_cond(p, cpu, def))
return false;
p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED);
+ p->sched_remote_delayed = !!(wake_flags & WF_DELAYED);
__ttwu_queue_wakelist(p, cpu);
return true;
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5327,7 +5327,10 @@ static __always_inline void return_cfs_r
static void set_delayed(struct sched_entity *se)
{
- se->sched_delayed = 1;
+ /*
+ * See TTWU_QUEUE_DELAYED in ttwu_runnable().
+ */
+ smp_store_release(&se->sched_delayed, 1);
/*
* Delayed se of cfs_rq have no tasks queued on them.
@@ -8481,6 +8484,18 @@ select_task_rq_fair(struct task_struct *
/* SD_flags and WF_flags share the first nibble */
int sd_flag = wake_flags & 0xF;
+ if (wake_flags & WF_DELAYED) {
+ /*
+ * This is the ttwu_delayed() case; where prev_cpu is in fact
+ * the CPU that did the wakeup, while @p is running on the
+ * current CPU.
+ *
+ * Make sure to flip them the right way around, otherwise
+ * wake-affine is going to do the wrong thing.
+ */
+ swap(cpu, new_cpu);
+ }
+
/*
* required for stable ->cpus_allowed
*/
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -82,6 +82,7 @@ SCHED_FEAT(TTWU_QUEUE, false)
SCHED_FEAT(TTWU_QUEUE, true)
#endif
SCHED_FEAT(TTWU_QUEUE_ON_CPU, true)
+SCHED_FEAT(TTWU_QUEUE_DELAYED, true)
SCHED_FEAT(TTWU_QUEUE_DEFAULT, false)
/*
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2280,6 +2280,7 @@ static inline int task_on_rq_migrating(s
#define WF_RQ_SELECTED 0x80 /* ->select_task_rq() was called */
#define WF_ON_CPU 0x0100
+#define WF_DELAYED 0x0200
static_assert(WF_EXEC == SD_BALANCE_EXEC);
static_assert(WF_FORK == SD_BALANCE_FORK);
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