[PATCH]sched: stop hrtick timer if running task is switching from fair scheduling class to another

[PATCH]sched: stop hrtick timer if running task is switching from fair scheduling class to another

[Kirill Tkhai]

[PATCH]sched: stop hrtick timer if running task is switching from fair
scheduling class to another

We have to stop hrtick timer to avoid excess interrupt. Not-fair tasks
are not interested in fair's hrtick. RT class uses its own fixed
timeslice (in case of RR), which doesn't depend on current value of hrtick
timer.

Kernel tree/version/git: next-20111216

This is resending, nobody answered me in previous time.

Signed-off-by: Kirill Tkhai <kirill.tkhai@gmail.com>
---

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 211cdc5..7733ba7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -368,7 +368,7 @@ static struct rq *this_rq_lock(void)
  * rq->lock.
  */
 
-static void hrtick_clear(struct rq *rq)
+void hrtick_clear(struct rq *rq)
 {
 	if (hrtimer_active(&rq->hrtick_timer))
 		hrtimer_cancel(&rq->hrtick_timer);
@@ -480,10 +480,6 @@ static void init_rq_hrtick(struct rq *rq)
 	rq->hrtick_timer.function = hrtick;
 }
 #else	/* CONFIG_SCHED_HRTICK */
-static inline void hrtick_clear(struct rq *rq)
-{
-}
-
 static inline void init_rq_hrtick(struct rq *rq)
 {
 }
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a4d2b7a..b083a91 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5271,6 +5271,13 @@ static void switched_from_fair(struct rq *rq,
struct task_struct *p)
 		place_entity(cfs_rq, se, 0);
 		se->vruntime -= cfs_rq->min_vruntime;
 	}
+
+	/*
+	 * Other scheduling classes are not interested in fair's hrtick timer.
+	 */
+	if (task_current(rq, p) && sched_feat(HRTICK))
+		hrtick_clear(rq);
+
 }
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d8d3613..f3c177c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -936,6 +936,8 @@ static inline int hrtick_enabled(struct rq *rq)
 	return hrtimer_is_hres_active(&rq->hrtick_timer);
 }
 
+void hrtick_clear(struct rq *rq);
+
 void hrtick_start(struct rq *rq, u64 delay);
 
 #else
@@ -945,6 +947,10 @@ static inline int hrtick_enabled(struct rq *rq)
 	return 0;
 }
 
+static inline void hrtick_clear(struct rq *rq)
+{
+}
+
 #endif /* CONFIG_SCHED_HRTICK */
 
 #ifdef CONFIG_SMP

Re: [PATCH]sched: stop hrtick timer if running task is switching from fair scheduling class to another

[Oleg Nesterov]

On 12/23, Kirill Tkhai wrote:
>
> [PATCH]sched: stop hrtick timer if running task is switching from fair
> scheduling class to another
>
> We have to stop hrtick timer to avoid excess interrupt. Not-fair tasks
> are not interested in fair's hrtick. RT class uses its own fixed
> timeslice (in case of RR), which doesn't depend on current value of hrtick
> timer.

Well. I shouldn't try to comment this patch, I do not really understand
this code.

But since nobody else replies...

> @@ -5271,6 +5271,13 @@ static void switched_from_fair(struct rq *rq,
> struct task_struct *p)
>  		place_entity(cfs_rq, se, 0);
>  		se->vruntime -= cfs_rq->min_vruntime;
>  	}
> +
> +	/*
> +	 * Other scheduling classes are not interested in fair's hrtick timer.
> +	 */
> +	if (task_current(rq, p) && sched_feat(HRTICK))
> +		hrtick_clear(rq);
> +
>  }

May be... but in this case, perhaps instead we should teach
dequeue_task_fair() or put_prev_task_fair() to do this. Then we can
probably remove __schedule()->hrtick_clear().

I simply can't understand the current hrtick logic. For example,
why dequeue_task_fair() does hrtick_update() ? OK, probably
because "nr_running < sched_nr_latency" can become true. But at
least this doesn't make sense to me when p == rq->curr, say,
__schedule() path.

Hmm. In any case, how it is possible to do hrtick_start() with
rq->lock held? hrtimer_restart() may want to wakeup_softirqd().

Peter, Ingo, does this code really work? SCHED_FEAT(HRTICK) == 0
by default, and afaics you can't change it without CONFIG_SCHED_DEBUG.

Confused...

Oleg.

Re: [PATCH]sched: stop hrtick timer if running task is switching from fair scheduling class to another

[Peter Zijlstra]

On Mon, 2011-12-26 at 16:42 +0100, Oleg Nesterov wrote:
> 
> Peter, Ingo, does this code really work? SCHED_FEAT(HRTICK) == 0
> by default, and afaics you can't change it without CONFIG_SCHED_DEBUG.
> 
> 

No its broken. Maybe I should've removed it, but I've always been
meaning to fix it since SCHED_DEADLINE actually needs it, but alas.
-ENOTIME and no urgency need have conspired against it.

SCHED_RR && time_slice

[Oleg Nesterov]

On 12/23, Kirill Tkhai wrote:
>
> RT class uses its own fixed
> timeslice (in case of RR),

which looks confusing too, btw.

sched_set_scheduler() does not initialize rt.time_slice, and
INIT_TASK() sets time_slice = HZ (not DEF_TIMESLICE).

Oleg.

Re: SCHED_RR && time_slice

[Kirill Tkhai]

Thanks for the answers, Oleg.

On Mon, 2011-12-26 at 17:59 +0100, Oleg Nesterov wrote:
> On 12/23, Kirill Tkhai wrote:
> >
> > RT class uses its own fixed
> > timeslice (in case of RR),
> 
> which looks confusing too, btw.
> 
> sched_set_scheduler() does not initialize rt.time_slice, and
> INIT_TASK() sets time_slice = HZ (not DEF_TIMESLICE).
> 

I talk about task_tick_rt(). When time_slice is over, it is being
renewed. New value is DEF_TIMESLICE. This happens in case of SCHED_RR
class.

Kirill

Re: SCHED_RR && time_slice

[Oleg Nesterov]

On 12/29, Kirill Tkhai wrote:
>
> Thanks for the answers, Oleg.
>
> On Mon, 2011-12-26 at 17:59 +0100, Oleg Nesterov wrote:
> > On 12/23, Kirill Tkhai wrote:
> > >
> > > RT class uses its own fixed
> > > timeslice (in case of RR),
> >
> > which looks confusing too, btw.
> >
> > sched_set_scheduler() does not initialize rt.time_slice, and
> > INIT_TASK() sets time_slice = HZ (not DEF_TIMESLICE).
> >
>
> I talk about task_tick_rt().

me too ;)

> When time_slice is over, it is being
> renewed. New value is DEF_TIMESLICE. This happens in case of SCHED_RR
> class.

Yes, but I was talking about initial value of time_slice, when the
task switches to SCHED_RR.

Oleg.

[PATCH]sched: stop hrtick timer if running task is switching from fair scheduling class to another

[Kirill Tkhai]

[PATCH]sched: stop hrtick timer if running task is switching from fair
scheduling class to another

We have to stop hrtick timer to avoid excess interrupt. Not-fair tasks
are not interested in fair's hrtick.

Kernel tree/version: next-20111216

Signed-off-by: Kirill Tkhai <tkhai@yandex.ru>
---

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 211cdc5..7733ba7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -368,7 +368,7 @@ static struct rq *this_rq_lock(void)
  * rq->lock.
  */
 
-static void hrtick_clear(struct rq *rq)
+void hrtick_clear(struct rq *rq)
 {
 	if (hrtimer_active(&rq->hrtick_timer))
 		hrtimer_cancel(&rq->hrtick_timer);
@@ -480,10 +480,6 @@ static void init_rq_hrtick(struct rq *rq)
 	rq->hrtick_timer.function = hrtick;
 }
 #else	/* CONFIG_SCHED_HRTICK */
-static inline void hrtick_clear(struct rq *rq)
-{
-}
-
 static inline void init_rq_hrtick(struct rq *rq)
 {
 }
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a4d2b7a..b083a91 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5271,6 +5271,13 @@ static void switched_from_fair(struct rq *rq,
struct task_struct *p)
 		place_entity(cfs_rq, se, 0);
 		se->vruntime -= cfs_rq->min_vruntime;
 	}
+
+	/*
+	 * Other scheduling classes are not interested in fair's hrtick timer.
+	 */
+	if (task_current(rq, p) && sched_feat(HRTICK))
+		hrtick_clear(rq);
+
 }
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d8d3613..f3c177c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -936,6 +936,8 @@ static inline int hrtick_enabled(struct rq *rq)
 	return hrtimer_is_hres_active(&rq->hrtick_timer);
 }
 
+void hrtick_clear(struct rq *rq);
+
 void hrtick_start(struct rq *rq, u64 delay);
 
 #else
@@ -945,6 +947,10 @@ static inline int hrtick_enabled(struct rq *rq)
 	return 0;
 }
 
+static inline void hrtick_clear(struct rq *rq)
+{
+}
+
 #endif /* CONFIG_SCHED_HRTICK */
 
 #ifdef CONFIG_SMP