Re: [PATCH v4] trace,x86: add x86 irq vector tracepoints

["H. Peter Anvin" <hpa@zytor.com>]

On 09/27/2012 03:33 PM, Seiji Aguchi wrote:
> Hi,
> 
>> ... except the cost can be reduced to zero *AND* be made into a more general mechanism by simply hooking the IDT.
> 
> Thank you for giving me the comment.
> In my understanding,  we can introduce a more general mechanism by sandwiching an existing handler between tracepoints.
> The pseudo code is like this:
> 
> @@ -17,7 +18,7 @@ static void default_threshold_interrupt(void)
>  
>  void (*mce_threshold_vector)(void) = default_threshold_interrupt;
>  
> -asmlinkage void smp_threshold_interrupt(void)
> +static void do_smp_threshold_interrupt(void)
>  {
>  	irq_enter();
>  	exit_idle();
> @@ -27,3 +28,10 @@ asmlinkage void smp_threshold_interrupt(void)
>  	/* Ack only at the end to avoid potential reentry */
>  	ack_APIC_irq();
>  }
> +
> +asmlinkage void smp_threshold_interrupt(void) {
> +	trace_arch_irq_vector_entry(THRESHOLD_APIC_VECTOR);
> +	do_smp_threshold_interrupt();
> +	trace_arch_irq_vector_exit(THRESHOLD_APIC_VECTOR);
> +}
> 
> If I misunderstand something, please let me know.
> 

Quite.

These functions are being invoked from the IDT, which is an indirect
pointer structure.  When not being traced, there is absolutely no reason
why it should go through a thunk with tracepoints.

	-hpa