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[PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock_task()

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  [PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock_task() Xunlei Pang <xlpang@redhat.com> - 2016-05-10 15:10 +0200
    Re: [PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock_task() bsegall@google.com - 2016-05-10 20:20 +0200
      Re: [PATCH] sched/fair: Fix the wrong throttled clock time for  cfs_rq_clock_task() Peter Zijlstra <peterz@infradead.org> - 2016-05-11 08:50 +0200
        Re: [PATCH] sched/fair: Fix the wrong throttled clock time for  cfs_rq_clock_task() Xunlei Pang <xpang@redhat.com> - 2016-05-12 05:40 +0200

#1398073 — [PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock_task()

Two minor fixes for cfs_rq_clock_task().
1) If cfs_rq is currently being throttled, we need to subtract the cfs
   throttled clock time.

2) Make "throttled_clock_task_time" update SMP unrelated. Now UP cases
   need it as well.

Signed-off-by: Xunlei Pang <xlpang@redhat.com>
---
 kernel/sched/fair.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1708729e..fb80a12 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3655,7 +3655,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
 static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq)
 {
 	if (unlikely(cfs_rq->throttle_count))
-		return cfs_rq->throttled_clock_task;
+		return cfs_rq->throttled_clock_task - cfs_rq->throttled_clock_task_time;
 
 	return rq_clock_task(rq_of(cfs_rq)) - cfs_rq->throttled_clock_task_time;
 }
@@ -3793,13 +3793,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
 	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
 
 	cfs_rq->throttle_count--;
-#ifdef CONFIG_SMP
 	if (!cfs_rq->throttle_count) {
 		/* adjust cfs_rq_clock_task() */
 		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
 					     cfs_rq->throttled_clock_task;
 	}
-#endif
 
 	return 0;
 }
-- 
1.8.3.1

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#1398375

Xunlei Pang <xlpang@redhat.com> writes:

> Two minor fixes for cfs_rq_clock_task().
> 1) If cfs_rq is currently being throttled, we need to subtract the cfs
>    throttled clock time.
>
> 2) Make "throttled_clock_task_time" update SMP unrelated. Now UP cases
>    need it as well.
>
> Signed-off-by: Xunlei Pang <xlpang@redhat.com>
> ---
>  kernel/sched/fair.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1708729e..fb80a12 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3655,7 +3655,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
>  static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq)
>  {
>  	if (unlikely(cfs_rq->throttle_count))
> -		return cfs_rq->throttled_clock_task;
> +		return cfs_rq->throttled_clock_task - cfs_rq->throttled_clock_task_time;
>  
>  	return rq_clock_task(rq_of(cfs_rq)) - cfs_rq->throttled_clock_task_time;
>  }
> @@ -3793,13 +3793,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
>  	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
>  
>  	cfs_rq->throttle_count--;
> -#ifdef CONFIG_SMP
>  	if (!cfs_rq->throttle_count) {
>  		/* adjust cfs_rq_clock_task() */
>  		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
>  					     cfs_rq->throttled_clock_task;
>  	}
> -#endif
>  
>  	return 0;
>  }

[Cc: pjt@google.com]

This looks reasonable to me (at least the first part; I'm not
certain why the CONFIG_SMP ifdef was put in place).

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#1398716 — Re: [PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock_task()

On Tue, May 10, 2016 at 11:19:44AM -0700, bsegall@google.com wrote:
> Xunlei Pang <xlpang@redhat.com> writes:
> 
> > Two minor fixes for cfs_rq_clock_task().
> > 1) If cfs_rq is currently being throttled, we need to subtract the cfs
> >    throttled clock time.
> >
> > 2) Make "throttled_clock_task_time" update SMP unrelated. Now UP cases
> >    need it as well.
> >
> > Signed-off-by: Xunlei Pang <xlpang@redhat.com>
> > ---
> >  kernel/sched/fair.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1708729e..fb80a12 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3655,7 +3655,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
> >  static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq)
> >  {
> >  	if (unlikely(cfs_rq->throttle_count))
> > -		return cfs_rq->throttled_clock_task;
> > +		return cfs_rq->throttled_clock_task - cfs_rq->throttled_clock_task_time;
> >  
> >  	return rq_clock_task(rq_of(cfs_rq)) - cfs_rq->throttled_clock_task_time;
> >  }

The alternative is obviously to do the subtraction in
tg_throttle_down(), were we set ->throttled_clock_task.

> > @@ -3793,13 +3793,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
> >  	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> >  
> >  	cfs_rq->throttle_count--;
> > -#ifdef CONFIG_SMP
> >  	if (!cfs_rq->throttle_count) {
> >  		/* adjust cfs_rq_clock_task() */
> >  		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
> >  					     cfs_rq->throttled_clock_task;
> >  	}
> > -#endif
> >  
> >  	return 0;
> >  }
> 
> [Cc: pjt@google.com]
> 
> This looks reasonable to me (at least the first part; I'm not
> certain why the CONFIG_SMP ifdef was put in place).

64660c864f46 ("sched: Prevent interactions with throttled entities")

Introduced it, because at that time it was about updating shares, which
is only present on SMP. Then:

f1b17280efbd ("sched: Maintain runnable averages across throttled periods")

Added the clock thing inside it, and:

82958366cfea ("sched: Replace update_shares weight distribution with per-entity computation")

took out the shares update and left the clock update, resulting in the
current code.

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#1399656 — Re: [PATCH] sched/fair: Fix the wrong throttled clock time for cfs_rq_clock_task()

On 2016/05/11 at 14:49, Peter Zijlstra wrote:
> On Tue, May 10, 2016 at 11:19:44AM -0700, bsegall@google.com wrote:
>> Xunlei Pang <xlpang@redhat.com> writes:
>>
>>> Two minor fixes for cfs_rq_clock_task().
>>> 1) If cfs_rq is currently being throttled, we need to subtract the cfs
>>>    throttled clock time.
>>>
>>> 2) Make "throttled_clock_task_time" update SMP unrelated. Now UP cases
>>>    need it as well.
>>>
>>> Signed-off-by: Xunlei Pang <xlpang@redhat.com>
>>> ---
>>>  kernel/sched/fair.c | 4 +---
>>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 1708729e..fb80a12 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -3655,7 +3655,7 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
>>>  static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq)
>>>  {
>>>  	if (unlikely(cfs_rq->throttle_count))
>>> -		return cfs_rq->throttled_clock_task;
>>> +		return cfs_rq->throttled_clock_task - cfs_rq->throttled_clock_task_time;
>>>  
>>>  	return rq_clock_task(rq_of(cfs_rq)) - cfs_rq->throttled_clock_task_time;
>>>  }
> The alternative is obviously to do the subtraction in
> tg_throttle_down(), were we set ->throttled_clock_task.

It is possible, but throttled_clock_task is a timestamp, I think doing it here is semantically better.

>
>>> @@ -3793,13 +3793,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
>>>  	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
>>>  
>>>  	cfs_rq->throttle_count--;
>>> -#ifdef CONFIG_SMP
>>>  	if (!cfs_rq->throttle_count) {
>>>  		/* adjust cfs_rq_clock_task() */
>>>  		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
>>>  					     cfs_rq->throttled_clock_task;
>>>  	}
>>> -#endif
>>>  
>>>  	return 0;
>>>  }
>> [Cc: pjt@google.com]
>>
>> This looks reasonable to me (at least the first part; I'm not
>> certain why the CONFIG_SMP ifdef was put in place).
> 64660c864f46 ("sched: Prevent interactions with throttled entities")
>
> Introduced it, because at that time it was about updating shares, which
> is only present on SMP. Then:
>
> f1b17280efbd ("sched: Maintain runnable averages across throttled periods")
>
> Added the clock thing inside it, and:
>
> 82958366cfea ("sched: Replace update_shares weight distribution with per-entity computation")
>
> took out the shares update and left the clock update, resulting in the
> current code.
>
>

Thanks,
Xunlei

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