linux-kernel - Re: [RFC][PATCH v3] sched/rt: Use IPI to trigger RT task push migration instead of pulling

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Date:	Thu, 26 Feb 2015 10:58:13 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jörn Engel <joern@...estorage.com>
Subject: Re: [RFC][PATCH v3] sched/rt: Use IPI to trigger RT task push
 migration instead of pulling

On Wed, 25 Feb 2015 22:26:44 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> +static int find_next_push_cpu(struct rq *rq)
> +{
> +	struct rq *next_rq;
> +	int cpu;
> +
> +	while (1) {
> +		cpu = rto_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 (next_rq->rt.highest_prio.next <
> +		    rq->rt.highest_prio.curr &&

> +		    next_rq->rt.highest_prio.next <
> +		    rq->rt.highest_prio.next)

OK, this second comparison is not needed. I was looking at the code in
more detail, and see that the .curr is updated when a new task is
enqueued, and it does not represent the task currently running, but the
highest priority task that is on the rq.

I'll be sending a v4 soon with this update.

-- Steve

> +			break;
> +	}
> +
> +	return cpu;
> +}
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