linux-kernel - Re: [PATCH] sched/rt: Avoid sending an IPI to a CPU already doing a push

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Date:	Fri, 8 Jul 2016 16:51:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] sched/rt: Avoid sending an IPI to a CPU already doing a
 push

On Fri, Jun 24, 2016 at 11:26:13AM -0400, Steven Rostedt wrote:
> The IPI code now needs to call push_tasks() instead of just push_task() as
> it will not be receiving an IPI for each CPU that is requesting a PULL.

My brain just skidded on that, can you try again with a few more words?


> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index d5690b722691..165bcfdbd94b 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -100,6 +100,7 @@ void init_rt_rq(struct rt_rq *rt_rq)
>  	rt_rq->push_flags = 0;
>  	rt_rq->push_cpu = nr_cpu_ids;
>  	raw_spin_lock_init(&rt_rq->push_lock);
> +	atomic_set(&rt_rq->ipi_count, 0);
>  	init_irq_work(&rt_rq->push_work, push_irq_work_func);
>  #endif
>  #endif /* CONFIG_SMP */
> @@ -1917,6 +1918,10 @@ static int find_next_push_cpu(struct rq *rq)
>  			break;
>  		next_rq = cpu_rq(cpu);
>  
> +		/* If pushing was already started, ignore */
> +		if (atomic_read(&next_rq->rt.ipi_count))
> +			continue;
> +
>  		/* Make sure the next rq can push to this rq */
>  		if (next_rq->rt.highest_prio.next < rq->rt.highest_prio.curr)
>  			break;
> @@ -1955,6 +1960,7 @@ static void tell_cpu_to_push(struct rq *rq)
>  		return;
>  
>  	rq->rt.push_flags = RT_PUSH_IPI_EXECUTING;
> +	atomic_inc(&cpu_rq(cpu)->rt.ipi_count);
>  
>  	irq_work_queue_on(&rq->rt.push_work, cpu);
>  }
> @@ -1974,11 +1980,12 @@ static void try_to_push_tasks(void *arg)
>  
>  	rq = cpu_rq(this_cpu);
>  	src_rq = rq_of_rt_rq(rt_rq);
> +	WARN_ON_ONCE(!atomic_read(&rq->rt.ipi_count));
>  
>  again:
>  	if (has_pushable_tasks(rq)) {
>  		raw_spin_lock(&rq->lock);
> -		push_rt_task(rq);
> +		push_rt_tasks(rq);

Maybe as a comment around here?

>  		raw_spin_unlock(&rq->lock);
>  	}
>  
> @@ -2000,7 +2007,7 @@ again:
>  	raw_spin_unlock(&rt_rq->push_lock);
>  
>  	if (cpu >= nr_cpu_ids)
> -		return;
> +		goto out;
>  
>  	/*
>  	 * It is possible that a restart caused this CPU to be
> @@ -2011,7 +2018,10 @@ again:
>  		goto again;
>  
>  	/* Try the next RT overloaded CPU */
> +	atomic_inc(&cpu_rq(cpu)->rt.ipi_count);
>  	irq_work_queue_on(&rt_rq->push_work, cpu);
> +out:
> +	atomic_dec(&rq->rt.ipi_count);
>  }

I have a vague feeling we're duplicating state, but I can't seem to spot
it, maybe I'm wrong.

Looks about right, but could use a comment, this stuff is getting rather
subtle.