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.
I'm not sure I follow, IRQs disabled fully implies !preemptible. I don't
see how the model would be more pessimistic than reality if it were to
use this knowledge.
Any !0 preempt_count(), which very much includes (Hard)IRQ and SoftIRQ
counts, means non-preemptible.
