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

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Message-ID: <20170302153841.4a30097e@gandalf.local.home>
Date:   Thu, 2 Mar 2017 15:38:41 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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 Wed, 1 Mar 2017 09:37:01 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> 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.

I always forgot that with documentation, pronouns should be avoided.

"But if has_pushable_tasks is false when entering"

> 
> > +	 * this hard interrupt handler function, then to have it set would

", then to have it set to true 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);