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

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Message-ID: <20150420234550.GA3182@worktop.meeting.verilan.com>
Date:	Tue, 21 Apr 2015 01:45:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Xunlei Pang <xlpang@....com>, linux-kernel@...r.kernel.org,
	Juri Lelli <juri.lelli@...il.com>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: Re: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for
 equal prio cases

On Mon, Apr 20, 2015 at 01:48:03PM -0400, Steven Rostedt wrote:
> On Mon, 20 Apr 2015 19:20:48 +0200
> Peter Zijlstra <peterz@...radead.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.

Please don't try and be clever there :-) Task state can be misleading,
you might get a wakeup before you're running again, in which case you
never went to sleep.

Please use task_on_rq_queued(p) like all other sites.
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