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

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Date:	Thu, 27 Sep 2012 19:25:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Seiji Aguchi <seiji.aguchi@....com>
CC:	"Thomas Gleixner (tglx@...utronix.de)" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"'mingo@...e.hu' (mingo@...e.hu)" <mingo@...e.hu>,
	"x86@...nel.org" <x86@...nel.org>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Satoru Moriya <satoru.moriya@....com>
Subject: Re: [PATCH v4] trace,x86: add x86 irq vector tracepoints

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



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