Re: [PATCH v3 1/4] tracing/preemptirq: Optimize preempt_disable/enable() tracepoint overhead

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 11, 2026 at 09:50:15AM -0300, Wander Lairson Costa wrote:

> +extern void __trace_preempt_on(void);
> +extern void __trace_preempt_off(void);
> +
> +DECLARE_TRACEPOINT(preempt_enable);
> +DECLARE_TRACEPOINT(preempt_disable);
> +
> +#define __preempt_trace_enabled(type, val) \
> +	(tracepoint_enabled(preempt_##type) && preempt_count() == (val))
> +
> +static __always_inline void preempt_count_add(int val)
> +{
> +	__preempt_count_add(val);
> +
> +	if (__preempt_trace_enabled(disable, val))
> +		__trace_preempt_off();
> +}
> +
> +static __always_inline void preempt_count_sub(int val)
> +{
> +	if (__preempt_trace_enabled(enable, val))
> +		__trace_preempt_on();
> +
> +	__preempt_count_sub(val);
> +}
>  #else
>  #define preempt_count_add(val)	__preempt_count_add(val)
>  #define preempt_count_sub(val)	__preempt_count_sub(val)
>  #define preempt_count_dec_and_test() __preempt_count_dec_and_test()
>  #endif
>  
> +#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
> +#define preempt_count_dec_and_test() \
> +	({ preempt_count_sub(1); should_resched(0); })
> +#endif

Why!?!

Why can't you simply have:

static __always_inline bool preempt_count_dec_and_test(void)
{
	if (__preempt_trace_enabled(enable, 1))
		__trace_preempt_on();

	return __preempt_count_dec_and_test();
}

Also, given how !x86 architectures were just complaining about how
terrible their preempt_emable() is, I'm really not liking this much at
all.

Currently the x86 preempt_disable() is _1_ instruction and
preempt_enable() is all of 3. Adding in these tracepoints will bloat
every single such site by at least another 4-5.

That's significant bloat, for really very little gain. Realistically
nobody is going to need these.