Date:	Mon, 30 Mar 2015 09:41:40 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/deadline: Use IPI to trigger DL task push
 migration instead of pulling

On Mon, 30 Mar 2015 07:07:10 +0800
Wanpeng Li <wanpeng.li@...ux.intel.com> wrote:

> +static int find_next_push_cpu(struct rq *rq)
> +{
> +	struct rq *next_rq;
> +	int cpu;
> +
> +	while (1) {
> +		cpu = dlo_next_cpu(rq);
> +		if (cpu >= nr_cpu_ids)
> +			break;
> +		next_rq = cpu_rq(cpu);
> +
> +		/* Make sure the next rq can push to this rq */
> +		if (dl_time_before(next_rq->dl.earliest_dl.next,
> +			rq->dl.earliest_dl.curr))
> +			break;
> +	}
> +
> +	return cpu;
> +}
> +

Is it possible that we don't duplicate the code and that we can find a
way to share the code between rt and dl? It's not that trivial to just
cut and paste. If a bug is found in one, it most likely wont be ported
to the other.

The best is if we can share the code here some way. Perhaps have a
single IPI that checks both rt and dl?

-- Steve

> +#define RT_PUSH_IPI_EXECUTING		1
> +#define RT_PUSH_IPI_RESTART		2
> +
> +static void tell_cpu_to_push(struct rq *rq)
> +{
> +	int cpu;
> +
> +	if (rq->dl.push_flags & RT_PUSH_IPI_EXECUTING) {
> +		raw_spin_lock(&rq->dl.push_lock);
> +		/* Make sure it's still executing */
> +		if (rq->dl.push_flags & RT_PUSH_IPI_EXECUTING) {
> +			/*
> +			 * Tell the IPI to restart the loop as things have
> +			 * changed since it started.
> +			 */
> +			rq->dl.push_flags |= RT_PUSH_IPI_RESTART;
> +			raw_spin_unlock(&rq->dl.push_lock);
> +			return;
> +		}
> +		raw_spin_unlock(&rq->dl.push_lock);
> +	}
> +
> +	/* When here, there's no IPI going around */
> +
> +	rq->dl.push_cpu = rq->cpu;
> +	cpu = find_next_push_cpu(rq);
> +	if (cpu >= nr_cpu_ids)
> +		return;
> +
> +	rq->dl.push_flags = RT_PUSH_IPI_EXECUTING;
> +
> +	irq_work_queue_on(&rq->dl.push_work, cpu);
> +}
> +
> +/* Called from hardirq context */
> +static void try_to_push_tasks(void *arg)
> +{
> +	struct dl_rq *dl_rq = arg;
> +	struct rq *rq, *src_rq;
> +	int this_cpu;
> +	int cpu;
> +
> +	this_cpu = dl_rq->push_cpu;
> +
> +	/* Paranoid check */
> +	BUG_ON(this_cpu != smp_processor_id());
> +
> +	rq = cpu_rq(this_cpu);
> +	src_rq = rq_of_dl_rq(dl_rq);
> +
> +again:
> +	if (has_pushable_dl_tasks(rq)) {
> +		raw_spin_lock(&rq->lock);
> +		push_dl_task(rq);
> +		raw_spin_unlock(&rq->lock);
> +	}
> +
> +	/* Pass the IPI to the next rt overloaded queue */
> +	raw_spin_lock(&dl_rq->push_lock);
> +	/*
> +	 * If the source queue changed since the IPI went out,
> +	 * we need to restart the search from that CPU again.
> +	 */
> +	if (dl_rq->push_flags & RT_PUSH_IPI_RESTART) {
> +		dl_rq->push_flags &= ~RT_PUSH_IPI_RESTART;
> +		dl_rq->push_cpu = src_rq->cpu;
> +	}
> +
> +	cpu = find_next_push_cpu(src_rq);
> +
> +	if (cpu >= nr_cpu_ids)
> +		dl_rq->push_flags &= ~RT_PUSH_IPI_EXECUTING;
> +	raw_spin_unlock(&dl_rq->push_lock);
> +
> +	if (cpu >= nr_cpu_ids)
> +		return;
> +
> +	/*
> +	 * It is possible that a restart caused this CPU to be
> +	 * chosen again. Don't bother with an IPI, just see if we
> +	 * have more to push.
> +	 */
> +	if (unlikely(cpu == rq->cpu))
> +		goto again;
> +
> +	/* Try the next RT overloaded CPU */
> +	irq_work_queue_on(&dl_rq->push_work, cpu);
> +}
> +
> +static void push_irq_work_func(struct irq_work *work)
> +{
> +	struct dl_rq *dl_rq = container_of(work, struct dl_rq, push_work);
> +
> +	try_to_push_tasks(dl_rq);
> +}
> +#endif /* HAVE_RT_PUSH_IPI */
> +
>  static int pull_dl_task(struct rq *this_rq)
>  {
>  	int this_cpu = this_rq->cpu, ret = 0, cpu;
> @@ -1432,6 +1602,13 @@ static int pull_dl_task(struct rq *this_rq)
>  	 */
>  	smp_rmb();
>  
> +#ifdef HAVE_RT_PUSH_IPI
> +	if (sched_feat(RT_PUSH_IPI)) {
> +		tell_cpu_to_push(this_rq);
> +		return 0;
> +	}
> +#endif
> +
>  	for_each_cpu(cpu, this_rq->rd->dlo_mask) {
>  		if (this_cpu == cpu)
>  			continue;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index dd532c5..87a937c 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -500,6 +500,12 @@ struct dl_rq {
>  	 */
>  	struct rb_root pushable_dl_tasks_root;
>  	struct rb_node *pushable_dl_tasks_leftmost;
> +#ifdef HAVE_RT_PUSH_IPI
> +	int push_flags;
> +	int push_cpu;
> +	struct irq_work push_work;
> +	raw_spinlock_t push_lock;
> +#endif
>  #else
>  	struct dl_bw dl_bw;
>  #endif

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