Re: [patch 3/21] x86, bts: wait until traced task has been scheduled out

[Markus Metzger <markus.t.metzger@googlemail.com>]

On Wed, 2009-04-01 at 21:04 +0200, Oleg Nesterov wrote:
> On 04/01, Metzger, Markus T wrote:
> >
> > >-----Original Message-----
> > >From: Oleg Nesterov [mailto:oleg@redhat.com]
> > >Sent: Wednesday, April 01, 2009 2:17 AM
> > >To: Metzger, Markus T
> >
> > >> +static void wait_to_unschedule(struct task_struct *task)
> > >> +{
> > >> +	unsigned long nvcsw;
> > >> +	unsigned long nivcsw;
> > >> +
> > >> +	if (!task)
> > >> +		return;
> > >> +
> > >> +	if (task == current)
> > >> +		return;
> > >> +
> > >> +	nvcsw  = task->nvcsw;
> > >> +	nivcsw = task->nivcsw;
> > >> +	for (;;) {
> > >> +		if (!task_is_running(task))
> > >> +			break;
> > >> +		/*
> > >> +		 * The switch count is incremented before the actual
> > >> +		 * context switch. We thus wait for two switches to be
> > >> +		 * sure at least one completed.
> > >> +		 */
> > >> +		if ((task->nvcsw - nvcsw) > 1)
> > >> +			break;
> > >> +		if ((task->nivcsw - nivcsw) > 1)
> > >> +			break;
> > >> +
> > >> +		schedule();
> > >
> > >schedule() is a nop here. We can wait unpredictably long...
> >
> > Hmmm, As far as I understand the code, rt-workqueues use a higher sched_class
> > and can thus not be preempted by normal threads. Non-rt workqueues
> > use the fair_sched_class. And schedule_work() uses a non-rt workqueue.
> 
> I was unclear, sorry.
> 
> I meant, in this case
> 
> 	while (!CONDITION)
> 		schedule();
> 
> is not better compared to
> 
> 	while (!CONDITION)
> 		; /* do nothing */
> 
> (OK, schedule() is better without CONFIG_PREEMPT, but this doesn't matter).
> wait_to_unschedule() just spins waiting for ->nXvcsw, this is not optimal.
> 
> And another problem, we can wait unpredictably long, because
> 
> > In practice, task is ptraced. It is either stopped or exiting.
> > I don't expect to loop very often.
> 
> No. The task _was_ ptraced when we called (say) ptrace_detach(). But when
> work->func() runs, the tracee is not traced, it is running (not necessary
> of course, the tracer _can_ leave it in TASK_STOPPED).
> 
> Now, again, suppose that this task does "for (;;) ;" in user-space.
> If CPU is "free", it can spin "forever" without re-scheduling. Yes sure,
> this case is not likely in practice, but still.

So I should rather not call schedule()?

I thought it's better to yield the cpu than to spin.


I will resend a bisect-friendly version of the series (using quilt mail,
this time) tomorrow.

I will remove schedule() in the wait_to_unschedule() loop and also
address the minor nitpicks you mentioned in your other reviews.

thanks,
markus.