[PATCH] sched: Remove broken check for skip clock update

[PATCH] sched: Remove broken check for skip clock update

[Frederic Weisbecker]

rq->skip_clock_update shouldn't be negative. Thus the check
in put_prev_task() is useless.

It was probably intended to do the following check:

	if (prev->on_rq && !rq->skip_clock_update)

We only want to update the clock if the current task is
not voluntarily sleeping: otherwise deactivate_task()
already did the rq clock update in schedule(). But we want
to ignore that update if a ttwu did it for us, in which case
rq->skip_clock_update is 1.

But update_rq_clock() already takes care of that so we
can just remove the broken condition.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/sched/core.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 15ba35e..8dfc461 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2886,7 +2886,7 @@ static inline void schedule_debug(struct task_struct *prev)
 
 static void put_prev_task(struct rq *rq, struct task_struct *prev)
 {
-	if (prev->on_rq || rq->skip_clock_update < 0)
+	if (prev->on_rq)
 		update_rq_clock(rq);
 	prev->sched_class->put_prev_task(rq, prev);
 }
-- 
1.7.5.4

Re: [PATCH] sched: Remove broken check for skip clock update

[Steven Rostedt]

On Tue, 2013-01-08 at 02:07 +0100, Frederic Weisbecker wrote:
> rq->skip_clock_update shouldn't be negative. Thus the check
> in put_prev_task() is useless.
> 
> It was probably intended to do the following check:
> 
> 	if (prev->on_rq && !rq->skip_clock_update)
> 
> We only want to update the clock if the current task is
> not voluntarily sleeping: otherwise deactivate_task()
> already did the rq clock update in schedule(). But we want
> to ignore that update if a ttwu did it for us, in which case
> rq->skip_clock_update is 1.
> 
> But update_rq_clock() already takes care of that so we
> can just remove the broken condition.

kernel/sched/rt.c:

                                /*
                                 * Force a clock update if the CPU was idle,
                                 * lest wakeup -> unthrottle time accumulate.
                                 */
                                if (rt_rq->rt_nr_running && rq->curr == rq->idle)
                                        rq->skip_clock_update = -1;


-- Steve

> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/sched/core.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 15ba35e..8dfc461 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2886,7 +2886,7 @@ static inline void schedule_debug(struct task_struct *prev)
>  
>  static void put_prev_task(struct rq *rq, struct task_struct *prev)
>  {
> -	if (prev->on_rq || rq->skip_clock_update < 0)
> +	if (prev->on_rq)
>  		update_rq_clock(rq);
>  	prev->sched_class->put_prev_task(rq, prev);
>  }

Re: [PATCH] sched: Remove broken check for skip clock update

[Frederic Weisbecker]

2013/1/8 Steven Rostedt <rostedt@goodmis.org>:
> On Tue, 2013-01-08 at 02:07 +0100, Frederic Weisbecker wrote:
>> rq->skip_clock_update shouldn't be negative. Thus the check
>> in put_prev_task() is useless.
>>
>> It was probably intended to do the following check:
>>
>>       if (prev->on_rq && !rq->skip_clock_update)
>>
>> We only want to update the clock if the current task is
>> not voluntarily sleeping: otherwise deactivate_task()
>> already did the rq clock update in schedule(). But we want
>> to ignore that update if a ttwu did it for us, in which case
>> rq->skip_clock_update is 1.
>>
>> But update_rq_clock() already takes care of that so we
>> can just remove the broken condition.
>
> kernel/sched/rt.c:
>
>                                 /*
>                                  * Force a clock update if the CPU was idle,
>                                  * lest wakeup -> unthrottle time accumulate.
>                                  */
>                                 if (rt_rq->rt_nr_running && rq->curr == rq->idle)
>                                         rq->skip_clock_update = -1;

Grr, I knew I was missing something!

Ok let's drop that patch then.