[patch] sched: call resched_task() conditionally from new task wake up path.

[patch] sched: call resched_task() conditionally from new task wake up path.

[Bharata B Rao]

Hi,

sched: call resched_task() conditionally from new task wake up path.

- During wake up of a new task, task_new_fair() can do a resched_task()
  on the current task. Later in the code path, check_preempt_curr() also ends
  up doing the same, which can be avoided. Check if TIF_NEED_RESCHED is
  already set for the current task.

- task_new_fair() does a resched_task() on the current task unconditionally.
  This can be done only in case when child runs before the parent.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
CC: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Ingo Molnar <mingo@elte.hu>
---
 kernel/sched_fair.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1348,6 +1348,13 @@ static void check_preempt_wakeup(struct 
 	if (unlikely(se == pse))
 		return;
 
+	/*
+	 * We can come here with TIF_NEED_RESCHED already set from new task
+	 * wake up path.
+	 */
+	if (test_tsk_need_resched(curr))
+		return;
+
 	cfs_rq_of(pse)->next = pse;
 
 	/*
@@ -1615,10 +1622,10 @@ static void task_new_fair(struct rq *rq,
 		 * 'current' within the tree based on its new key value.
 		 */
 		swap(curr->vruntime, se->vruntime);
+		resched_task(rq->curr);
 	}
 
 	enqueue_task_fair(rq, p, 0);
-	resched_task(rq->curr);
 }
 
 /*

Regards,
Bharata.

Re: [patch] sched: call resched_task() conditionally from new task wake up path.

[Peter Zijlstra]

On Thu, 2008-08-28 at 14:42 +0530, Bharata B Rao wrote:
> Hi,
> 
> sched: call resched_task() conditionally from new task wake up path.
> 
> - During wake up of a new task, task_new_fair() can do a resched_task()
>   on the current task. Later in the code path, check_preempt_curr() also ends
>   up doing the same, which can be avoided. Check if TIF_NEED_RESCHED is
>   already set for the current task.
> 
> - task_new_fair() does a resched_task() on the current task unconditionally.
>   This can be done only in case when child runs before the parent.
> 
> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> CC: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> CC: Ingo Molnar <mingo@elte.hu>
> ---
>  kernel/sched_fair.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1348,6 +1348,13 @@ static void check_preempt_wakeup(struct 
>  	if (unlikely(se == pse))
>  		return;
>  
> +	/*
> +	 * We can come here with TIF_NEED_RESCHED already set from new task
> +	 * wake up path.
> +	 */
> +	if (test_tsk_need_resched(curr))
> +		return;
> +
>  	cfs_rq_of(pse)->next = pse;
>  
>  	/*
> @@ -1615,10 +1622,10 @@ static void task_new_fair(struct rq *rq,
>  		 * 'current' within the tree based on its new key value.
>  		 */
>  		swap(curr->vruntime, se->vruntime);
> +		resched_task(rq->curr);
>  	}
>  
>  	enqueue_task_fair(rq, p, 0);
> -	resched_task(rq->curr);
>  }
>  
>  /*
> 
> Regards,
> Bharata.

Re: [patch] sched: call resched_task() conditionally from new task wake up path.

[Ingo Molnar]

* Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:

> Hi,
> 
> sched: call resched_task() conditionally from new task wake up path.
> 
> - During wake up of a new task, task_new_fair() can do a resched_task()
>   on the current task. Later in the code path, check_preempt_curr() also ends
>   up doing the same, which can be avoided. Check if TIF_NEED_RESCHED is
>   already set for the current task.
> 
> - task_new_fair() does a resched_task() on the current task unconditionally.
>   This can be done only in case when child runs before the parent.
> 
> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>

well spotted - applied to tip/sched/devel, thanks Bharata.

	Ingo