[tip: sched/core] sched: Prepare generic code for delayed dequeue

tip-bot2 for Peter Zijlstra
2024-08-18T06:23:13Z

The following commit has been merged into the sched/core branch of tip:

Commit-ID: abc158c82ae555078aa5dd2d8407c3df0f868904
Gitweb: https://git.kernel.org/tip/abc158c82ae555078aa5dd2d8407c3df0f868904
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 23 May 2024 10:55:59 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Sat, 17 Aug 2024 11:06:42 +02:00

sched: Prepare generic code for delayed dequeue

While most of the delayed dequeue code can be done inside the
sched_class itself, there is one location where we do not have an
appropriate hook, namely ttwu_runnable().

Add an ENQUEUE_DELAYED call to the on_rq path to deal with waking
delayed dequeue tasks.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Tested-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lkml.kernel.org/r/20240727105029.200000445@infradead.org
---
include/linux/sched.h | 1 +
kernel/sched/core.c | 14 +++++++++++++-
kernel/sched/sched.h | 2 ++
3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2c1b4ee..f4a648e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -544,6 +544,7 @@ struct sched_entity {

struct list_head group_node;
unsigned int on_rq;
+ unsigned int sched_delayed;

u64 exec_start;
u64 sum_exec_runtime;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6c59548..7356464 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2036,6 +2036,8 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags)

void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
{
+ SCHED_WARN_ON(flags & DEQUEUE_SLEEP);
+
WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING);
ASSERT_EXCLUSIVE_WRITER(p->on_rq);

@@ -3689,12 +3691,14 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)

rq = __task_rq_lock(p, &rf);
if (task_on_rq_queued(p)) {
+ update_rq_clock(rq);
+ if (p->se.sched_delayed)
+ enqueue_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_DELAYED);
if (!task_on_cpu(rq, p)) {
/*
* When on_rq && !on_cpu the task is preempted, see if
* it should preempt the task that is current now.
*/
- update_rq_clock(rq);
wakeup_preempt(rq, p, wake_flags);
}
ttwu_do_wakeup(p);
@@ -4074,11 +4078,16 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
* case the whole 'p->on_rq && ttwu_runnable()' case below
* without taking any locks.
*
+ * Specifically, given current runs ttwu() we must be before
+ * schedule()'s block_task(), as such this must not observe
+ * sched_delayed.
+ *
* In particular:
* - we rely on Program-Order guarantees for all the ordering,
* - we're serialized against set_special_state() by virtue of
* it disabling IRQs (this allows not taking ->pi_lock).
*/
+ SCHED_WARN_ON(p->se.sched_delayed);
if (!ttwu_state_match(p, state, &success))
goto out;

@@ -4370,6 +4379,9 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
p->se.slice = sysctl_sched_base_slice;
INIT_LIST_HEAD(&p->se.group_node);

+ /* A delayed task cannot be in clone(). */
+ SCHED_WARN_ON(p->se.sched_delayed);
+
#ifdef CONFIG_FAIR_GROUP_SCHED
p->se.cfs_rq = NULL;
#endif
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 69ab3b0..ffca977 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2253,6 +2253,7 @@ extern const u32 sched_prio_to_wmult[40];
#define DEQUEUE_MOVE 0x04 /* Matches ENQUEUE_MOVE */
#define DEQUEUE_NOCLOCK 0x08 /* Matches ENQUEUE_NOCLOCK */
#define DEQUEUE_MIGRATING 0x100 /* Matches ENQUEUE_MIGRATING */
+#define DEQUEUE_DELAYED 0x200 /* Matches ENQUEUE_DELAYED */

#define ENQUEUE_WAKEUP 0x01
#define ENQUEUE_RESTORE 0x02
@@ -2268,6 +2269,7 @@ extern const u32 sched_prio_to_wmult[40];
#endif
#define ENQUEUE_INITIAL 0x80
#define ENQUEUE_MIGRATING 0x100
+#define ENQUEUE_DELAYED 0x200

#define RETRY_TASK ((void *)-1UL)