linux-kernel - Re: [PATCH] sched/rt: Document why has_pushable_tasks() isn't called with a runqueue lock

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Message-ID: <20170301083701.GX6515@twins.programming.kicks-ass.net>
Date:   Wed, 1 Mar 2017 09:37:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] sched/rt: Document why has_pushable_tasks() isn't called
 with a runqueue lock

On Tue, Feb 28, 2017 at 04:48:56PM -0500, Steven Rostedt wrote:

> +	/*
> +	 * Normally, has_pushable_tasks() would be performed within the
> +	 * runqueue lock being held. But if it was not set when entering

"not set" what? I'm having trouble parsing this.

> +	 * this hard interrupt handler function, then to have it set would
> +	 * require a wake up. A wake up of an RT task will either cause a
> +	 * schedule if the woken task is higher priority than the running
> +	 * task, or it would try to do a push from the CPU doing the wake
> +	 * up. Grabbing the runqueue lock in such a case would more likely
> +	 * just cause unnecessary contention.
> +	 */
>  	if (has_pushable_tasks(rq)) {
>  		raw_spin_lock(&rq->lock);
>  		push_rt_task(rq);