[PATCH RFC v5 3/4] Revert "sched/rt: Substract number of tasks of throttled queues from rq->nr_running"
From: Qais Yousef
Date: Mon Nov 02 2026 - 20:00:47 EST
This partially reverts commit f4ebcbc0d7e009783256c9daf76bc4b90e645c14.
While trying to address uclamp static key race condition the throttling
logic in RT and fair has highlighted that both perform incomplete
dequeue since they don't go through core. RT logic seemed to diverge
from fair pattern where rq->nr_running was management oddly.
Digging into the history highlighted this partially reverted patch has
added this out-of-line management of rq->nr_running instead in the usual
place when tasks are actually added or removed from internal
structures. It also had a confusing call in sched_rt_rq_dequeue() to
dequeue_top_rt_rq() when it is essentially a dead code - in existing
code base at least.
Revert the logic, but don't break the functionality. Instead do {add,
sub}_nr_running() in {inc, dec}_rt_tasks() where the rest of the
housekeeping is done.
Drop rt_queued usage in rt_rq_is_runnable() since it was unnecessary
check. And make sched_rt_is_runnable() an alias to rt_rq_is_runnable()
since we no longer have rt_queued and both functions are essentially
identical.
Signed-off-by: Qais Yousef <qyousef@xxxxxxxxxxx>
---
kernel/sched/rt.c | 74 ++++++--------------------------------------
kernel/sched/sched.h | 6 ++--
2 files changed, 11 insertions(+), 69 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 1535046a23ffa..cefb0a8d37e81 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -82,8 +82,6 @@ void init_rt_rq(struct rt_rq *rt_rq)
rt_rq->highest_prio.next = MAX_RT_PRIO-1;
rt_rq->overloaded = 0;
plist_head_init(&rt_rq->pushable_tasks);
- /* We start is dequeued state, because no RT tasks are queued */
- rt_rq->rt_queued = 0;
#ifdef CONFIG_RT_GROUP_SCHED
rt_rq->rt_time = 0;
@@ -429,8 +427,6 @@ static void dequeue_pushable_task(struct rq *rq, struct task_struct *p)
}
}
-static void enqueue_top_rt_rq(struct rt_rq *rt_rq);
-static void dequeue_top_rt_rq(struct rt_rq *rt_rq, unsigned int count);
static inline int on_rt_rq(struct sched_rt_entity *rt_se)
{
@@ -535,9 +531,7 @@ static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
rt_se = rt_rq->tg->rt_se[cpu];
if (rt_rq->rt_nr_running) {
- if (!rt_se)
- enqueue_top_rt_rq(rt_rq);
- else if (!on_rt_rq(rt_se))
+ if (rt_se && !on_rt_rq(rt_se))
enqueue_rt_entity(rt_se, 0);
if (rt_rq->highest_prio.curr < donor->prio)
@@ -553,7 +547,6 @@ static void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
rt_se = rt_rq->tg->rt_se[cpu];
if (!rt_se) {
- dequeue_top_rt_rq(rt_rq, rt_rq->rt_nr_running);
/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
cpufreq_update_util(rq_of_rt_rq(rt_rq), 0);
}
@@ -920,18 +913,12 @@ static inline struct rt_rq *group_rt_rq(struct sched_rt_entity *rt_se)
static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
{
- struct rq *rq = rq_of_rt_rq(rt_rq);
-
- if (!rt_rq->rt_nr_running)
- return;
-
- enqueue_top_rt_rq(rt_rq);
- resched_curr(rq);
+ if (rt_rq->rt_nr_running)
+ resched_curr(rq_of_rt_rq(rt_rq));
}
static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
{
- dequeue_top_rt_rq(rt_rq, rt_rq->rt_nr_running);
}
static inline int rt_rq_throttled(struct rt_rq *rt_rq)
@@ -1007,44 +994,6 @@ static void update_curr_rt(struct rq *rq)
#endif /* CONFIG_RT_GROUP_SCHED */
}
-static void
-dequeue_top_rt_rq(struct rt_rq *rt_rq, unsigned int count)
-{
- struct rq *rq = rq_of_rt_rq(rt_rq);
-
- BUG_ON(&rq->rt != rt_rq);
-
- if (!rt_rq->rt_queued)
- return;
-
- BUG_ON(!rq->nr_running);
-
- sub_nr_running(rq, count);
- rt_rq->rt_queued = 0;
-
-}
-
-static void
-enqueue_top_rt_rq(struct rt_rq *rt_rq)
-{
- struct rq *rq = rq_of_rt_rq(rt_rq);
-
- BUG_ON(&rq->rt != rt_rq);
-
- if (rt_rq->rt_queued)
- return;
-
- if (rt_rq_throttled(rt_rq))
- return;
-
- if (rt_rq->rt_nr_running) {
- add_nr_running(rq, rt_rq->rt_nr_running);
- rt_rq->rt_queued = 1;
- }
-
- /* Kick cpufreq (see the comment in kernel/sched/sched.h). */
- cpufreq_update_util(rq, 0);
-}
static void
inc_rt_prio_smp(struct rt_rq *rt_rq, int prio, int prev_prio)
@@ -1182,6 +1131,9 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
inc_rt_prio(rt_rq, prio);
inc_rt_group(rt_se, rt_rq);
+
+ if (!group_rt_rq(rt_se))
+ add_nr_running(rq_of_rt_rq(rt_rq), 1);
}
static inline
@@ -1194,6 +1146,9 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
dec_rt_prio(rt_rq, rt_se_prio(rt_se));
dec_rt_group(rt_se, rt_rq);
+
+ if (!group_rt_rq(rt_se))
+ sub_nr_running(rq_of_rt_rq(rt_rq), 1);
}
/*
@@ -1383,39 +1338,29 @@ static void __dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flag
static void dequeue_rt_stack(struct sched_rt_entity *rt_se, unsigned int flags)
{
struct sched_rt_entity *back = NULL;
- unsigned int rt_nr_running;
for_each_sched_rt_entity(rt_se) {
rt_se->back = back;
back = rt_se;
}
- rt_nr_running = rt_rq_of_se(back)->rt_nr_running;
-
for (rt_se = back; rt_se; rt_se = rt_se->back) {
if (on_rt_rq(rt_se))
__dequeue_rt_entity(rt_se, flags);
}
-
- dequeue_top_rt_rq(rt_rq_of_se(back), rt_nr_running);
}
static void enqueue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
{
- struct rq *rq = rq_of_rt_se(rt_se);
-
update_stats_enqueue_rt(rt_rq_of_se(rt_se), rt_se, flags);
dequeue_rt_stack(rt_se, flags);
for_each_sched_rt_entity(rt_se)
__enqueue_rt_entity(rt_se, flags);
- enqueue_top_rt_rq(&rq->rt);
}
static void dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
{
- struct rq *rq = rq_of_rt_se(rt_se);
-
update_stats_dequeue_rt(rt_rq_of_se(rt_se), rt_se, flags);
dequeue_rt_stack(rt_se, flags);
@@ -1426,7 +1371,6 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
if (rt_rq && rt_rq->rt_nr_running)
__enqueue_rt_entity(rt_se, flags);
}
- enqueue_top_rt_rq(&rq->rt);
}
/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4706f73838a86..0cab55963657c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -872,8 +872,6 @@ struct rt_rq {
bool overloaded;
struct plist_head pushable_tasks;
- int rt_queued;
-
#ifdef CONFIG_RT_GROUP_SCHED
int rt_throttled;
u64 rt_time; /* consumed RT time, goes up in update_curr_rt */
@@ -892,7 +890,7 @@ struct rt_rq {
static inline bool rt_rq_is_runnable(struct rt_rq *rt_rq)
{
- return rt_rq->rt_queued && rt_rq->rt_nr_running;
+ return rt_rq->rt_nr_running;
}
/* Deadline class' related fields in a runqueue */
@@ -2918,7 +2916,7 @@ static inline bool sched_dl_runnable(struct rq *rq)
static inline bool sched_rt_runnable(struct rq *rq)
{
- return rq->rt.rt_queued > 0;
+ return rt_rq_is_runnable(&rq->rt);
}
static inline bool sched_fair_runnable(struct rq *rq)
--
2.53.0