linux-kernel - Re: [PATCH][RFC] tracing/context-tracking: Add preempt_schedule

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Date:	Fri, 31 May 2013 11:18:48 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH][RFC] tracing/context-tracking: Add
 preempt_schedule_context() for tracing

On Fri, 2013-05-31 at 15:43 +0200, Frederic Weisbecker wrote:

> > +void __sched notrace preempt_schedule_context(void)
> > +{
> > +	struct thread_info *ti = current_thread_info();
> > +	enum ctx_state prev_ctx;
> > +
> > +	if (likely(ti->preempt_count || irqs_disabled()))
> > +		return;
> > +
> > +	/*
> > +	 * Need to disable preemption in case user_exit() is traced
> > +	 * and the tracer calls preempt_enable_notrace() causing
> > +	 * an infinite recursion.
> > +	 */
> > +	preempt_disable_notrace();
> > +	prev_ctx = this_cpu_read(context_tracking.state);
> > +	user_exit();
> 
> You can reuse exception_enter()

I originally did use that, but then noticed that everything else in
context_tracking.c used context_tracking.state directly. I have no
problems doing it this way again.

> 
> > +	preempt_enable_no_resched_notrace();
> > +
> > +	preempt_schedule();
> > +
> > +	preempt_disable_notrace();
> > +	if (prev_ctx == IN_USER)
> > +		user_enter();
> 
> And then exception_exit() here.
> 
> I guess this replaces your fix with schedule_preempt_user(). I liked
> it because it seems that:
> 
>    if (need_resched()) {
>       user_exit();
>       local_irq_enable();
>       schedule();
>       local_irq_enable();
>       user_enter();
>    }
> 
> is a common pattern of arch user resume preemption that we can consolidate.
> 
> But your new patch probably makes it more widely safe for the function tracer
> for any function that can be called and traced in IN_USER mode. Not only user preemption.
> Think about do_notify_resume() for example if it is called after syscall_trace_leave().
> 
> Independantly, schedule_preempt_user() is still interesting for consolidation.

And I think that patch is still valid from just a clean up point of
view. It just didn't cover all the cases needed for tracing.

I'll rewrite the patch and send it out for another review.

Thanks!

-- Steve


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