linux-kernel - Re: [PATCH tip/sched/core v2] sched/rt: Simplify the IPI rt balancing logic

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Message-ID: <20170504110155.2ed7b158@gandalf.local.home>
Date:   Thu, 4 May 2017 11:01:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        John Kacur <jkacur@...hat.com>, Scott Wood <swood@...hat.com>
Subject: Re: [PATCH tip/sched/core v2] sched/rt: Simplify the IPI rt
 balancing logic

On Thu, 4 May 2017 16:41:28 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Apr 24, 2017 at 11:47:32AM -0400, Steven Rostedt wrote:
> > +	/* Keep the loop going if the IPI is currently active */
> > +	atomic_inc_return(&rq->rd->rto_loop_next);  
> 
> What's the return for?

-ENOCONTEXT (too much cut from email)

/me goes and finds his email that he sent.

	/* Keep the loop going if the IPI is currently active */
	atomic_inc_return(&rq->rd->rto_loop_next);

	/* Only one CPU can initiate a loop at a time */
	if (!rto_start_trylock(&rq->rd->rto_loop_start))
		return;

Ah, it's not needed. I think I had that to supply a full memory
barrier for a previous version, but now that rto_start_trylock() is:

static inline bool rto_start_trylock(atomic_t *v)
{
	return !atomic_cmpxchg(v, 0, 1);
}

Which supplies its own memory barrier, it can now be a simple

   atomic_inc().

Thanks, will update.

-- Steve