linux-kernel - Re: [PATCH v2][RFC] tracing/context-tracking: Add preempt_schedule

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Message-ID: <1370439709.9844.17.camel@gandalf.local.home>
Date:	Wed, 05 Jun 2013 09:41:49 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2][RFC] tracing/context-tracking: Add
 preempt_schedule_context() for tracing

On Wed, 2013-06-05 at 13:45 +0200, Peter Zijlstra wrote:
> On Tue, Jun 04, 2013 at 10:16:56AM -0400, Steven Rostedt wrote:
> > index 65349f0..73b89d9 100644
> > --- a/kernel/context_tracking.c
> > +++ b/kernel/context_tracking.c
> > @@ -71,6 +71,46 @@ void user_enter(void)
> >  	local_irq_restore(flags);
> >  }
> >  
> > +#ifdef CONFIG_PREEMPT
> > +/**
> > + * preempt_schedule_context - preempt_schedule called by tracing
> > + *
> > + * The tracing infrastructure uses preempt_enable_notrace to prevent
> > + * recursion and tracing preempt enabling caused by the tracing
> > + * infrastructure itself. But as tracing can happen in areas coming
> > + * from userspace or just about to enter userspace, a preempt enable
> > + * can occur before user_exit() is called. This will cause the scheduler
> > + * to be called when the system is still in usermode.
> > + *
> > + * To prevent this, the preempt_enable_notrace will use this function
> > + * instead of preempt_schedule() to exit user context if needed before
> > + * calling the scheduler.
> > + */
> 
> If preempt_enable_notrace() is the only user, why does this live in
> kernel/context_tracking.c and not in kernel/sched/core.c?

Then we would need to add #ifdef CONFIG_CONTEXT_TRACKING around it too.
As we have in preempt.h:

#ifdef CONFIG_CONTEXT_TRACKING

void preempt_schedule_context(void);

#define preempt_check_resched_context() \
do { \
	if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
		preempt_schedule_context(); \
} while(0)
#else

#define preempt_check_resched_context() preempt_check_resched()

#endif

Seemed like it was more appropriate to be in context_tracking.c than
sched/core.c, as it depends on context tracking.

> 
> > +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 = exception_enter();
> > +	preempt_enable_no_resched_notrace();
> > +
> > +	preempt_schedule();
> > +
> > +	preempt_disable_notrace();
> > +	exception_exit(prev_ctx);
> > +	preempt_enable_notrace();
> > +}
> > +EXPORT_SYMBOL(preempt_schedule_context);
> 
> Do we really need to export this?

As preempt_enable_notrace() is also used by tracepoints, yes. Because
tracepoints are used by modules. Hmm, now it may be exported via a GPL
export though.

-- Steve


> 
> > +#endif /* CONFIG_PREEMPT */
> >  
> >  /**
> >   * user_exit - Inform the context tracking that the CPU is
> > -- 
> > 1.7.10.4
> > 
> > 
> > 


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