On Mon, Feb 24, 2014 at 06:14:24PM +0800, Michael wang wrote:
> On 02/24/2014 03:10 PM, Peter Zijlstra wrote:
> > On Mon, Feb 24, 2014 at 01:19:15PM +0800, Michael wang wrote:
> >> Peter, do we accidentally missed this commit?
> >>
> >> http://git.kernel.org/tip/477af336ba06ef4c32e97892bb0d2027ce30f466
> >
> > Ingo dropped it on Saturday because it makes locking_selftest() unhappy.
> >
> > That is because we call locking_selftest() way before we're ready to
> > call schedule() and guess what it does :-/
> >
> > I'm not entirely sure what to do.. ideally I'd shoot locking_selftest in
> > the head, but clearly that's not entirely desired either.
>
> ...what about move idle_balance() back to it's old position?
I've always hated that, idle_balance() is very much a fair policy thing
and shouldn't live in the core code.
> pull_rt_task() logical could be after idle_balance() if still no FAIR
> and DL, then go into the pick loop, that may could make things more
> clean & clear, should we have a try?
So the reason pull_{rt,dl}_task() is before idle_balance() is that we
don't want to add the execution latency of idle_balance() to the rt/dl
task pulling.
Anyway, the below seems to work; it avoids playing tricks with the idle
thread and instead uses a magic constant.
The comparison should be faster too; seeing how we avoid dereferencing
p->sched_class.
---
Subject: sched: Guarantee task priority in pick_next_task()
From: Peter Zijlstra <pet...@infradead.org>
Date: Fri Feb 14 12:25:08 CET 2014
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}().
Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
Cc: Juri Lelli <juri.le...@gmail.com>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Steven Rostedt <rost...@goodmis.org>
Reported-by: Michael Wang <wang...@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <pet...@infradead.org>
---
kernel/sched/core.c | 12 ++++++++----
kernel/sched/fair.c | 13 ++++++++++++-
kernel/sched/rt.c | 10 +++++++++-
kernel/sched/sched.h | 5 +++++
4 files changed, 34 insertions(+), 6 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2586,24 +2586,28 @@ static inline void schedule_debug(struct
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 */
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4687,6 +4687,7 @@ pick_next_task_fair(struct rq *rq, struc
struct cfs_rq *cfs_rq = &rq->cfs;
struct sched_entity *se;
struct task_struct *p;
+ int new_tasks;
again:
#ifdef CONFIG_FAIR_GROUP_SCHED
@@ -4785,7 +4786,17 @@ pick_next_task_fair(struct rq *rq, struc
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;
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct
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;
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1090,6 +1090,8 @@ static const u32 prio_to_wmult[40] = {
#define DEQUEUE_SLEEP 1
+#define RETRY_TASK ((void *)-1UL)
+
struct sched_class {
const struct sched_class *next;
@@ -1104,6 +1106,9 @@ 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);
--
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