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

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Date:   Mon, 24 Apr 2017 10:51:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.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>
Subject: Re: [RFC][PATCH tip/sched/core] sched/rt: Simplify the IPI rt
 balancing logic

On Fri, Apr 21, 2017 at 10:49:29PM -0400, Steven Rostedt wrote:
> When a CPU schedules in a lower priority task and wants to make sure
> overloaded CPUs know about it. It increments the rto_loop_next. Then it does
> an atomic_inc_return() on rto_loop_start. If the returned value is not "1",
> then it does atomic_dec() on rt_loop_start and returns. If the value is "1",
> then it will take the rto_lock to synchronize with a possible IPI being sent
> around to the overloaded CPUs.

> +	start = atomic_inc_return(&rq->rd->rto_loop_start);
> +	if (start != 1)
> +		goto out;

> +out:
> +	atomic_dec(&rq->rd->rto_loop_start);


Did you just write a very expensive test-and-set trylock?