Re: Interrupt caused by software events

[Martin Ichilevici de Oliveira <martin.i.oliveira@gmail.com>]

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On Wed, Jul 09, 2014 at 10:05:55AM -0400, Vince Weaver wrote:
> Not really sure by what you mean by "real-time", but you can theoretically 
> set up the perf_event mmap buffer with a low threshold, and have it 
> configured to send a signal to your program whenever an event comes in.
> In theory you can get the PEBS-like info out that way.  It's going to have 
> fairly high overhead though, especially if you set the sample rate high.

On Wed, Jul 09, 2014 at 03:54:18PM +0200, Jiri Olsa wrote:
> hi,
> you could use perf_event_create_kernel_counter interface,
> check the lockup watchdog in kernel/watchdog.c

Vince and Jiri,

Thank you very much for your replies.

I re-read my email and I'm not sure I was clear, so just in case I
didn't made myself clear, here's a high-level example of what I'm
looking for.

/*******************/
/* Gets called whenever my program generates a cache-miss */
void my_func()
{
    unsigned long addr = get_ip_that_generated_event();
    printf("A cache-miss occurred at IP: %x!\n", addr);
}

int main()
{
    struct perf_event_attr pe;
    pe.type = PERF_TYPE_HARDWARE.
    pe.config = PERF_COUNT_HW_CACHE_MISSES;
    ...
    register_handler(&pe, my_func);
    // code to be monitored
    unregister_handler();
    return 0;
}
/*******************/

Every time an event occurs (a cache-miss in my example), my_func should
get called. The get_ip_that_generated_event() function should return the
Instruction Pointer that generated the event.

My impression is that this could be done with a low overhead (other then
that introduced by what I do in my_func), because 
"perf record -e cache-misses ./a.out"
can pinpoint with little overhead how many times the event was caused by
each instruction. But maybe I'm just missing something.

Vince:
I'm unsure how to configure the mmap buffer you mentioned, and I
couldn't find any examples online. Could you please point me to some
reference, or maybe provide a little example?

Jiri:
I'm under the impression I'd have to be in a kernel module (or in kernel
mode in general) to use perf_event_create_kernel_counter(). Is that
correct?

Thank you once again,
-Martin

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