Re: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases

From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Xunlei Pang <xlpang@126.com>,
	linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@gmail.com>,
	Xunlei Pang <pang.xunlei@linaro.org>
Subject: Re: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases
Date: Mon, 20 Apr 2015 13:48:03 -0400	[thread overview]
Message-ID: <20150420134803.5446e32d@gandalf.local.home> (raw)
In-Reply-To: <20150420172048.GZ27490@worktop.programming.kicks-ass.net>

On Mon, 20 Apr 2015 19:20:48 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> > > +		 */
> > > +		if (preempt_count() & PREEMPT_ACTIVE)
> > > +			enqueue_pushable_task_preempted(rq, p);
> > > +		else
> > > +			enqueue_pushable_task(rq, p);
> > > +	}
> > >  }
> 
> This looks wrong, what do you want to find? _any_ preemption? In that
> case PREEMPT_ACTIVE is wrong. What you need to check is if the task is
> still on the RQ or not.
> 
> If the task was put to sleep it got dequeued, if it was not dequeued, it
> got preempted.
> 
> PREEMPT_ACTIVE is only ever set for forced kernel preemption, which is a
> special sub case only ever triggered with CONFIG_PREEMPT=y.

Ah, you're right. I was thinking of just forced preemption, but, I
wasn't thinking about voluntary preemption (preemption points). We want
this behavior for that too (for kernel).

And yes, if we preempt in user space, this isn't enough either.

Actually, I think we only care if the state of the task is
TASK_RUNNING, if it is anything else, the task is probably going to
sleep anyway and we don't care about FIFO order then.

-- Steve