sched: Guarantee task priority in pick_next_task()

Michael spotted that the idle_balance() push down created a task
priority problem.

Previously, when we called idle_balance() before pick_next_task() it
wasn't a problem when -- because of the rq->lock droppage -- an rt/dl
task slipped in.

Similarly for pre_schedule(), rt pre-schedule could have a dl task
slip in.

But by pulling it into the pick_next_task() loop, we'll not try a
higher task priority again.

Cure this by creating a re-start condition in pick_next_task(); and
triggering this from pick_next_task_{rt,fair}().

It also fixes a live-lock where we get stuck in pick_next_task_fair()
due to idle_balance() seeing !0 nr_running but there not actually
being any fair tasks about.

Reported-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Fixes: 38033c37fa ("sched: Push down pre_schedule() and idle_balance()")
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20140224121218.GR15586@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2014-02-14 12:25:08 +01:00 committed by Ingo Molnar
parent 06d50c65b1
commit 37e117c07b
4 changed files with 34 additions and 6 deletions

View File

@ -2586,24 +2586,28 @@ static inline void schedule_debug(struct task_struct *prev)
static inline struct task_struct *
pick_next_task(struct rq *rq, struct task_struct *prev)
{
const struct sched_class *class;
const struct sched_class *class = &fair_sched_class;
struct task_struct *p;
/*
* Optimization: we know that if all tasks are in
* the fair class we can call that function directly:
*/
if (likely(prev->sched_class == &fair_sched_class &&
if (likely(prev->sched_class == class &&
rq->nr_running == rq->cfs.h_nr_running)) {
p = fair_sched_class.pick_next_task(rq, prev);
if (likely(p))
if (likely(p && p != RETRY_TASK))
return p;
}
again:
for_each_class(class) {
p = class->pick_next_task(rq, prev);
if (p)
if (p) {
if (unlikely(p == RETRY_TASK))
goto again;
return p;
}
}
BUG(); /* the idle class will always have a runnable task */

View File

@ -4686,6 +4686,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev)
struct cfs_rq *cfs_rq = &rq->cfs;
struct sched_entity *se;
struct task_struct *p;
int new_tasks;
again:
#ifdef CONFIG_FAIR_GROUP_SCHED
@ -4784,7 +4785,17 @@ simple:
return p;
idle:
if (idle_balance(rq)) /* drops rq->lock */
/*
* Because idle_balance() releases (and re-acquires) rq->lock, it is
* possible for any higher priority task to appear. In that case we
* must re-start the pick_next_entity() loop.
*/
new_tasks = idle_balance(rq);
if (rq->nr_running != rq->cfs.h_nr_running)
return RETRY_TASK;
if (new_tasks)
goto again;
return NULL;

View File

@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
struct task_struct *p;
struct rt_rq *rt_rq = &rq->rt;
if (need_pull_rt_task(rq, prev))
if (need_pull_rt_task(rq, prev)) {
pull_rt_task(rq);
/*
* pull_rt_task() can drop (and re-acquire) rq->lock; this
* means a dl task can slip in, in which case we need to
* re-start task selection.
*/
if (unlikely(rq->dl.dl_nr_running))
return RETRY_TASK;
}
if (!rt_rq->rt_nr_running)
return NULL;

View File

@ -1091,6 +1091,8 @@ static const u32 prio_to_wmult[40] = {
#define DEQUEUE_SLEEP 1
#define RETRY_TASK ((void *)-1UL)
struct sched_class {
const struct sched_class *next;
@ -1105,6 +1107,9 @@ struct sched_class {
* It is the responsibility of the pick_next_task() method that will
* return the next task to call put_prev_task() on the @prev task or
* something equivalent.
*
* May return RETRY_TASK when it finds a higher prio class has runnable
* tasks.
*/
struct task_struct * (*pick_next_task) (struct rq *rq,
struct task_struct *prev);