Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt_counter change

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, May 29, 2019 at 11:40:34AM +0200, Daniel Bristot de Oliveira wrote:
> On 29/05/2019 10:33, Peter Zijlstra wrote:
> > On Tue, May 28, 2019 at 05:16:23PM +0200, Daniel Bristot de Oliveira wrote:
> >> The preempt_disable/enable tracepoint only traces in the disable <-> enable
> >> case, which is correct. But think about this case:
> >>
> >> ---------------------------- %< ------------------------------
> >> 	THREAD					IRQ
> >> 	   |					 |
> >> preempt_disable() {
> >>     __preempt_count_add(1)
> >> 	------->	    smp_apic_timer_interrupt() {
> >> 				preempt_disable()
> >> 				    do not trace (preempt count >= 1)
> >> 				    ....
> >> 				preempt_enable()
> >> 				    do not trace (preempt count >= 1)
> >> 			    }
> >>     trace_preempt_disable();
> >> }
> >> ---------------------------- >% ------------------------------
> >>
> >> The tracepoint will be skipped.
> > 
> > .... for the IRQ. But IRQs are not preemptible anyway, so what the
> > problem?
> 
> 
> right, they are.
> 
> exposing my problem in a more specific way:
> 
> To show in a model that an event always takes place with preemption disabled,
> but not necessarily with IRQs disabled, it is worth having the preemption
> disable events separated from IRQ disable ones.
> 
> The main reason is that, although IRQs disabled postpone the execution of the
> scheduler, it is more pessimistic, as it also delays IRQs. So the more precise
> the model is, the less pessimistic the analysis will be.
> 
> But there are other use-cases, for instance:
> 
> (Steve, correct me if I am wrong)
> 
> The preempt_tracer will not notice a "preempt disabled" section in an IRQ
> handler if the problem above happens.
> 
> (Yeah, I know these problems are very specific... but...)

I agree with the problem. I think Daniel does not want to miss the preemption
disabled event caused by the IRQ disabling.

> >> To avoid skipping the trace, the change in the counter should be "atomic"
> >> with the start/stop, w.r.t the interrupts.
> >>
> >> Disable interrupts while the adding/starting stopping/subtracting.
> > 
> >> +static inline void preempt_add_start_latency(int val)
> >> +{
> >> +	unsigned long flags;
> >> +
> >> +	raw_local_irq_save(flags);
> >> +	__preempt_count_add(val);
> >> +	preempt_latency_start(val);
> >> +	raw_local_irq_restore(flags);
> >> +}
> > 
> >> +static inline void preempt_sub_stop_latency(int val)
> >> +{
> >> +	unsigned long flags;
> >> +
> >> +	raw_local_irq_save(flags);
> >> +	preempt_latency_stop(val);
> >> +	__preempt_count_sub(val);
> >> +	raw_local_irq_restore(flags);
> >> +}
> > 
> > That is hideously expensive :/
> 
> Yeah... :-( Is there another way to provide such "atomicity"?
> 
> Can I use the argument "if one has these tracepoints enabled, they are not
> considering it as a hot-path?"

The only addition here seems to  the raw_local_irq_{save,restore} around the
calls to increment the preempt counter and start the latency tracking.

Is there any performance data with the tracepoint enabled and with/without
this patch? Like with hackbench?

Thanks.