[RFC patch powerpc,trace] Avoid suspicious RCU usage reporting for some tracepoints

[Steven Rostedt]

On Mon, 2012-09-10 at 15:10 +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2012-09-10 at 12:58 +0800, Li Zhong wrote:
> > There are a few tracepoints in the interrupt code path, which is before
> > irq_enter(), or after irq_exit(), like
> > trace_irq_entry()/trace_irq_exit() in do_IRQ(),
> > trace_timer_interrupt_entry()/trace_timer_interrupt_exit() in
> > timer_interrupt(). 
> > 
> > If the interrupt is from idle(), and because tracepoint contains RCU
> > read-side critical section, we could see following suspicious RCU usage
> > reported:
> 
>  .../...
> 
> > This is because the RCU usage in interrupt context should be used in
> > area marked by rcu_irq_enter()/rcu_irq_exit(), called in
> > irq_enter()/irq_exit() respectively. 
> > 
> > Could we add a new tracepoint trace_***_rcuirq, like trace_***_rcuidle
> > to avoid the report? like the code attached below.
> > 
> > Or could we just move these tracepoints inside the
> > irq_enter()/irq_exit() area? (Seems not good for the timer_interrupt
> > case). 
> 
> I'd say just move them in. Anton, any objection ?
> 

I agree too. I'm a bit concerned about the lack of coverage the
irq_enter() and irq_exit() have in the timer interrupt:

	may_hard_irq_enable();

	trace_timer_interrupt_entry(regs);

	__get_cpu_var(irq_stat).timer_irqs++;

#if defined(CONFIG_PPC32) && defined(CONFIG_PMAC)
	if (atomic_read(&ppc_n_lost_interrupts) != 0)
		do_IRQ(regs);
#endif

	old_regs = set_irq_regs(regs);
	irq_enter();


I'm guessing that call to do_IRQ() has to do with lazy irq handling?

Anyway, there may be a reason to have the tracepoint before this call,
but I'm not sure it really is that important. It should probably be best
to move it after the irq_enter(). There's side-effects with calling
things from interrupt context outside of irq_enter/exit().

-- Steve