Re: [PATCH v2] sched: push rt tasks only if newly activatedtaskshave been added

[Gregory Haskins]

>>> On Mon, Apr 21, 2008 at 12:20 PM, in message <20080421162032.GA12645@xxxxxxx>,
Ingo Molnar <mingo@xxxxxxx> wrote: 

> got a build failure:
> 
>  In file included from kernel/sched.c:1972:
>  kernel/sched_rt.c: In function 'enqueue_task_rt':
>  kernel/sched_rt.c:522: error: 'struct rt_rq' has no member named 'pushed'
> 
> with this config:
> 
>  http://redhat.com/~mingo/misc/config-Mon_Apr_21_18_02_45_CEST_2008.bad
> 
> with the patch below against sched-devel/latest.

Ah, sorry.  I had written it against 2.6.25 so I suspect a fuzz issue.   I will apply to sched-devel and resolve the errors.

> 
> 	Ingo
> 
> ----------->
> Subject: sched: push rt tasks only if newly activated tasks have been added
> From: Gregory Haskins <ghaskins@xxxxxxxxxx>
> Date: Mon, 21 Apr 2008 11:42:18 -0400
> 
> SCHED_RR can context-switch many times without having changed the
> run-queue. Therefore trying to push on each context switch can just be
> wasted effort since if it failed the first time, it will likely fail any
> subsequent times as well.  Instead, set a flag when we have successfully
> pushed as many tasks away as possible, and only clear it when the
> runqueue adds new tasks (effectively becoming a run-queue "dirty bit").
> If new tasks are added we should try again.  If any remote run-queues
> downgrade their priority in the meantime, they will try to pull from us
> (as they always do).
> 
> This attempts to address a performance regression reported by Suresh
> Siddha, et. al. in the 2.6.25 series.
> 
> Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
> CC: suresh.b.siddha@xxxxxxxxx
> CC: mingo@xxxxxxx
> CC: rostedt@xxxxxxxxxxx
> CC: chinang.ma@xxxxxxxxx
> CC: arjan@xxxxxxxxxxxxxxx
> CC: willy@xxxxxxxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
> ---
>  kernel/sched.c    |    2 ++
>  kernel/sched_rt.c |   22 ++++++++++++++++++++--
>  2 files changed, 22 insertions(+), 2 deletions(-)
> 
> Index: linux/kernel/sched.c
> ===================================================================
> --- linux.orig/kernel/sched.c
> +++ linux/kernel/sched.c
> @@ -462,6 +462,7 @@ struct rt_rq {
>  #ifdef CONFIG_SMP
>  	unsigned long rt_nr_migratory;
>  	int overloaded;
> +	int pushed;
>  #endif
>  	int rt_throttled;
>  	u64 rt_time;
> @@ -8073,6 +8074,7 @@ static void init_rt_rq(struct rt_rq *rt_
>  #ifdef CONFIG_SMP
>  	rt_rq->rt_nr_migratory = 0;
>  	rt_rq->overloaded = 0;
> +	rt_rq->pushed = 0;
>  #endif
>  
>  	rt_rq->rt_time = 0;
> Index: linux/kernel/sched_rt.c
> ===================================================================
> --- linux.orig/kernel/sched_rt.c
> +++ linux/kernel/sched_rt.c
> @@ -514,6 +514,13 @@ static void enqueue_task_rt(struct rq *r
>  	for_each_sched_rt_entity(rt_se)
>  		enqueue_rt_entity(rt_se);
>  
> +	/*
> +	 * Clear the "pushed" state since we have changed the run-queue and
> +	 * may need to migrate some of these tasks (see push_rt_tasks() for
> +	 * details)
> +	 */
> +	rq->rt.pushed = 0;
> +
>  	inc_cpu_load(rq, p->se.load.weight);
>  }
>  
> @@ -913,7 +920,7 @@ static int push_rt_task(struct rq *rq)
>  	int ret = 0;
>  	int paranoid = RT_MAX_TRIES;
>  
> -	if (!rq->rt.overloaded)
> +	if (!rq->rt.overloaded || rq->rt.pushed)
>  		return 0;
>  
>  	next_task = pick_next_highest_task_rt(rq, -1);
> @@ -973,6 +980,15 @@ out:
>  }
>  
>  /*
> + * push_rt_tasks()
> + *
> + * Push as many tasks away as possible.  We only need to try once whenever
> + * one or more new tasks are added to our runqueue.  After an inital 
> attempt,
> + * further changes in remote runqueue state will be accounted for with pull
> + * operations.  Therefore, we mark the run-queue as "pushed" here, and clear 
> it
> + * during enqueue.  This primarily helps SCHED_RR tasks which will tend to
> + * have a higher context-switch to enqueue ratio.
> + *
>   * TODO: Currently we just use the second highest prio task on
>   *       the queue, and stop when it can't migrate (or there's
>   *       no more RT tasks).  There may be a case where a lower
> @@ -987,6 +1003,8 @@ static void push_rt_tasks(struct rq *rq)
>  	/* push_rt_task will return true if it moved an RT */
>  	while (push_rt_task(rq))
>  		;
> +
> +	rq->rt.pushed = 1;
>  }
>  
>  static int pull_rt_task(struct rq *this_rq)
> @@ -1091,7 +1109,7 @@ static void post_schedule_rt(struct rq *
>  	 * the lock was owned by prev, we need to release it
>  	 * first via finish_lock_switch and then reaquire it here.
>  	 */
> -	if (unlikely(rq->rt.overloaded)) {
> +	if (unlikely(rq->rt.overloaded && !rq->rt.pushed)) {
>  		spin_lock_irq(&rq->lock);
>  		push_rt_tasks(rq);
>  		spin_unlock_irq(&rq->lock);
> --
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