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

[Peter Zijlstra <peterz@infradead.org>]

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 :/