[RFC PATCH] sched: make sure sched-priority after invoke idle_balance()

[RFC PATCH] sched: make sure sched-priority after invoke idle_balance()

[Michael wang]

Since idle_balance() will release rq-lock for a while, there is a chance that
RT/DL tasks will be enqueued and ask for the resched, the func used to be
invoked ahead of pick_next_task(), which will make sure we drop into the
bottom-half inside pick_next_task().

Now since idle_balance() was done inside pick_next_task_fair(), pick_next_task()
can no longer make sure the priority, the worst case is that we will going to
pick the pulled fair task while there is RT/DL on rq which actually should be
picked up.

This patch will prevent this happen by some rechecking after idle_balance(), it
utilize the resched-flag for the case when RT/DL task was enqueued but don't ask
for resched (will that ever happened?).

CC: Ingo Molnar <mingo@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
---
 kernel/sched/fair.c |   23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 235cfa7..ce67514 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4776,6 +4776,16 @@ simple:
 
 idle:
 #ifdef CONFIG_SMP
+	/*
+	 * We came here only when there is no more tasks on rq (top-half of
+	 * pick_next_task()), and we are now going to pull some fair entities.
+	 *
+	 * Since prev is still the current on rq, clear it's resched-flag so
+	 * we would be able to know when we got a new resched-request during
+	 * idle_balance(), check below for more details.
+	 */
+	clear_tsk_need_resched(prev);
+
 	idle_enter_fair(rq);
 	/*
 	 * We must set idle_stamp _before_ calling idle_balance(), such that we
@@ -4784,7 +4794,18 @@ idle:
 	rq->idle_stamp = rq_clock(rq);
 	if (idle_balance(rq)) { /* drops rq->lock */
 		rq->idle_stamp = 0;
-		goto again;
+		/*
+		 * Before we start to pick one of the pulled fair entities, take
+		 * care if some RT/DL tasks has been enqueued during the time
+		 * we release rq-lock inside idle_balance().
+		 *
+		 * In such cases, since clear_tsk_need_resched() was done
+		 * already, need_resched() will imply the request to sched-in
+		 * the enqueued RT/DL tasks, so don't 'goto again' to make sure
+		 * the priority.
+		 */
+		if (rq->nr_running == rq->cfs.h_nr_running || !need_resched())
+			goto again;
 	}
 #endif
 
-- 
1.7.9.5

Re: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance()

[Peter Zijlstra]

On Fri, Feb 14, 2014 at 12:54:37PM +0800, Michael wang wrote:
> Since idle_balance() will release rq-lock for a while, there is a chance that
> RT/DL tasks will be enqueued and ask for the resched, the func used to be
> invoked ahead of pick_next_task(), which will make sure we drop into the
> bottom-half inside pick_next_task().
> 
> Now since idle_balance() was done inside pick_next_task_fair(), pick_next_task()
> can no longer make sure the priority, the worst case is that we will going to
> pick the pulled fair task while there is RT/DL on rq which actually should be
> picked up.
> 
> This patch will prevent this happen by some rechecking after idle_balance(), it
> utilize the resched-flag for the case when RT/DL task was enqueued but don't ask
> for resched (will that ever happened?).

I'm not sure this is actually working right; the problem is that while
we do retry on need_resched() in the main schedule() loop, that last
need_resched() is on @next (then current). So clearing/resetting @prev's
need_resched() is not going to trigger that loop.

Not to mention we explicitly clear @prev's need_resched right after
pick_next_task().

So how about something like this?

I don't particularly like adding that condition to pick_next_task(); but
the alternative is recursively calling pick_next_task() and while
recursion is strictly limited to the number of sched_classes, it does
feel kinda icky.

Anybody got any preferences?

---
Subject: sched: Guarantee task priority in pick_next_task()
From: Peter Zijlstra <peterz@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.lelli@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-jrdk7auga87duk4lkpo8xusk@git.kernel.org
---
 kernel/sched/core.c |   19 +++++++++++++++----
 kernel/sched/fair.c |   16 +++++++++++++++-
 kernel/sched/rt.c   |   10 +++++++++-
 3 files changed, 39 insertions(+), 6 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2574,27 +2574,38 @@ 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))
-			return p;
+			goto got_task;
 	}
 
+again:
 	for_each_class(class) {
 		p = class->pick_next_task(rq, prev);
 		if (p)
-			return p;
+			goto got_task;
 	}
 
 	BUG(); /* the idle class will always have a runnable task */
+
+got_task:
+	/*
+	 * See pick_next_task_{fair,rt}(); they return rq->idle in case
+	 * they want to re-start the task selection.
+	 */
+	if (unlikely(p->sched_class != class))
+		goto again;
+
+	return p;
 }
 
 /*
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4684,6 +4684,7 @@ 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
@@ -4782,7 +4783,20 @@ 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);
+
+	/*
+	 * See pick_next_task(); we return rq->idle to restart task selection.
+	 */
+	if (rq->nr_running != rq->cfs.h_nr_running)
+		return rq->idle;
+
+	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 rq->idle;
+	}
 
 	if (!rt_rq->rt_nr_running)
 		return NULL;

Re: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance()

[Michael wang]

On 02/14/2014 08:38 PM, Peter Zijlstra wrote:
[snip]
>>
>> This patch will prevent this happen by some rechecking after idle_balance(), it
>> utilize the resched-flag for the case when RT/DL task was enqueued but don't ask
>> for resched (will that ever happened?).
> 
> I'm not sure this is actually working right; the problem is that while
> we do retry on need_resched() in the main schedule() loop, that last
> need_resched() is on @next (then current). So clearing/resetting @prev's
> need_resched() is not going to trigger that loop.
> 
> Not to mention we explicitly clear @prev's need_resched right after
> pick_next_task().

Actually it's not aim at that timing, but consider about the RT case, it
won't work as expected anyway...

> 
> So how about something like this?
> 
> I don't particularly like adding that condition to pick_next_task(); but
> the alternative is recursively calling pick_next_task() and while
> recursion is strictly limited to the number of sched_classes, it does
> feel kinda icky.

Yeah...but it works, the RT stuff is inside the loop and really hard to
be handled...

> 
> Anybody got any preferences?
> 
> ---
> Subject: sched: Guarantee task priority in pick_next_task()
[snip]
>  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))
> -			return p;
> +			goto got_task;

Since idle_balance() won't happen in the loop, may be we could use:

	if p && p->sched_class == class
		return p

in here, let it fall down into the loop if p is idle, since that means
we got RT/DL and will do this anyway, could save two jump work may be?
(and may could combine some code below if so?)

Regards,
Michael Wang

>  	}
> 
> +again:
>  	for_each_class(class) {
>  		p = class->pick_next_task(rq, prev);
>  		if (p)
> -			return p;
> +			goto got_task;
>  	}
> 
>  	BUG(); /* the idle class will always have a runnable task */
> +
> +got_task:
> +	/*
> +	 * See pick_next_task_{fair,rt}(); they return rq->idle in case
> +	 * they want to re-start the task selection.
> +	 */
> +	if (unlikely(p->sched_class != class))
> +		goto again;
> +
> +	return p;
>  }
> 
>  /*
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4684,6 +4684,7 @@ 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
> @@ -4782,7 +4783,20 @@ 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);
> +
> +	/*
> +	 * See pick_next_task(); we return rq->idle to restart task selection.
> +	 */
> +	if (rq->nr_running != rq->cfs.h_nr_running)
> +		return rq->idle;
> +
> +	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 rq->idle;
> +	}
> 
>  	if (!rt_rq->rt_nr_running)
>  		return NULL;
> --
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Re: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance()

[Peter Zijlstra]

On Mon, Feb 17, 2014 at 11:31:16AM +0800, Michael wang wrote:
> >  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))
> > -			return p;
> > +			goto got_task;
> 
> Since idle_balance() won't happen in the loop, may be we could use:
> 
> 	if p && p->sched_class == class
> 		return p
> 
> in here, let it fall down into the loop if p is idle, since that means
> we got RT/DL and will do this anyway, could save two jump work may be?
> (and may could combine some code below if so?)

Maybe; we'd have to look at whatever GCC does with it. But yes I think I
like the code better that way.

Re: [RFC PATCH] sched: make sure sched-priority after invoke idle_balance()

[Michael wang]

On 02/17/2014 07:24 PM, Peter Zijlstra wrote:
[snip]
>> Since idle_balance() won't happen in the loop, may be we could use:
>>
>> 	if p && p->sched_class == class
>> 		return p
>>
>> in here, let it fall down into the loop if p is idle, since that means
>> we got RT/DL and will do this anyway, could save two jump work may be?
>> (and may could combine some code below if so?)
> 
> Maybe; we'd have to look at whatever GCC does with it. 

Exactly, alien code appear when in binary...

Regards,
Michael Wang

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