[RFC PATCH v3 04/10] sched/fair: Introduce sched_throttle_work

[Valentin Schneider]

Later commits will change CFS bandwidth control throttling from a
per-cfs_rq basis to a per-task basis. Actual throttling of a task will
happen in the return to user path, which will be implemented via a
task_work callback.

To ease reviewing, the infrastructure and helpers are added first, the
actual behaviour will be implemented when switching to per-task
throttling.

Add a task_work node to struct task_struct, and have it initialised at
sched_fork().

Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
 include/linux/sched.h |  1 +
 kernel/sched/core.c   |  4 ++++
 kernel/sched/fair.c   | 12 ++++++++++++
 kernel/sched/sched.h  |  2 ++
 4 files changed, 19 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 90691d99027e3..a4976eb5065fc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -812,6 +812,7 @@ struct task_struct {
 
 #ifdef CONFIG_CGROUP_SCHED
 	struct task_group		*sched_task_group;
+	struct callback_head            sched_throttle_work;
 #endif
 
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6d35c48239be0..b811670d2c362 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4329,6 +4329,10 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	p->se.cfs_rq			= NULL;
 #endif
 
+#ifdef CONFIG_CFS_BANDWIDTH
+	init_cfs_throttle_work(p);
+#endif
+
 #ifdef CONFIG_SCHEDSTATS
 	/* Even if schedstat is disabled, there should not be garbage */
 	memset(&p->stats, 0, sizeof(p->stats));
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9057584ec06de..775547cdd3ce0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5759,6 +5759,18 @@ static int tg_throttle_down(struct task_group *tg, void *data)
 	return 0;
 }
 
+static void throttle_cfs_rq_work(struct callback_head *work)
+{
+
+}
+
+void init_cfs_throttle_work(struct task_struct *p)
+{
+	/* Protect against double add, see throttle_cfs_rq() and throttle_cfs_rq_work() */
+	p->sched_throttle_work.next = &p->sched_throttle_work;
+	init_task_work(&p->sched_throttle_work, throttle_cfs_rq_work);
+}
+
 static bool throttle_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	struct rq *rq = rq_of(cfs_rq);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4c36cc6803617..943bca8263ffe 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2510,6 +2510,8 @@ extern void init_sched_dl_class(void);
 extern void init_sched_rt_class(void);
 extern void init_sched_fair_class(void);
 
+extern void init_cfs_throttle_work(struct task_struct *p);
+
 extern void reweight_task(struct task_struct *p, const struct load_weight *lw);
 
 extern void resched_curr(struct rq *rq);
-- 
2.43.0