[Devel] Re: Re: Hang with fair cgroup scheduler (reproducer is attached.)

[Dmitry Adamushko]

Ingo,

what about the following patch instead?

maybe task_is_current() would be a better name though.

Steven,

I guess, there is some analogue of UNLOCKED_CTXSW on -rt
(to reduce contention for rq->lock).
So there can be a race schedule() vs. rt_mutex_setprio() or sched_setscheduler()
for some paths that might explain crashes you have been observing?

I haven't analyzed this case for -rt, so I'm just throwing in the idea in case it can be useful.


---------------------------------------------------

From: Dmitry Adamushko <dmitry.adamushko at gmail.com>

sched: introduce task_current()

Some services (e.g. sched_setscheduler(), rt_mutex_setprio() and sched_move_task())
must handle a given task differently in case it's the 'rq->curr' task on its run-queue.
The task_running() interface is not suitable for determining such tasks
for platforms with one of the following options:

#define __ARCH_WANT_UNLOCKED_CTXSW
#define __ARCH_WANT_INTERRUPTS_ON_CTXSW

Due to the fact that it makes use of 'p->oncpu == 1' as a criterion but
such a task is not necessarily 'rq->curr'.

The detailed explanation is available here:
https://lists.linux-foundation.org/pipermail/containers/2007-December/009262.html


Signed-off-by: Dmitry Adamushko <dmitry.adamushko at gmail.com>

---
diff --git a/kernel/sched.c b/kernel/sched.c
index dc6fb24..15d088b 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -619,10 +619,15 @@ EXPORT_SYMBOL_GPL(cpu_clock);
 # define finish_arch_switch(prev)	do { } while (0)
 #endif
 
+static inline int task_current(struct rq *rq, struct task_struct *p)
+{
+	return rq->curr == p;
+}
+
 #ifndef __ARCH_WANT_UNLOCKED_CTXSW
 static inline int task_running(struct rq *rq, struct task_struct *p)
 {
-	return rq->curr == p;
+	return task_current(rq, p);
 }
 
 static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
@@ -651,7 +656,7 @@ static inline int task_running(struct rq *rq, struct task_struct *p)
 #ifdef CONFIG_SMP
 	return p->oncpu;
 #else
-	return rq->curr == p;
+	return task_current(rq, p);
 #endif
 }
 
@@ -3340,7 +3345,7 @@ unsigned long long task_sched_runtime(struct task_struct *p)
 
 	rq = task_rq_lock(p, &flags);
 	ns = p->se.sum_exec_runtime;
-	if (rq->curr == p) {
+	if (task_current(rq, p)) {
 		update_rq_clock(rq);
 		delta_exec = rq->clock - p->se.exec_start;
 		if ((s64)delta_exec > 0)
@@ -4033,7 +4038,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 
 	oldprio = p->prio;
 	on_rq = p->se.on_rq;
-	running = task_running(rq, p);
+	running = task_current(rq, p);
 	if (on_rq) {
 		dequeue_task(rq, p, 0);
 		if (running)
@@ -4334,7 +4339,7 @@ recheck:
 	}
 	update_rq_clock(rq);
 	on_rq = p->se.on_rq;
-	running = task_running(rq, p);
+	running = task_current(rq, p);
 	if (on_rq) {
 		deactivate_task(rq, p, 0);
 		if (running)
@@ -7360,7 +7365,7 @@ void sched_move_task(struct task_struct *tsk)
 
 	update_rq_clock(rq);
 
-	running = task_running(rq, tsk);
+	running = task_current(rq, tsk);
 	on_rq = tsk->se.on_rq;
 
 	if (on_rq) {

---

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