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?
> 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 :/
