[RFC PATCH 0/1] Notifications for perf sideband events

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Currently, there is no way to ask for signals to be delivered when a
certain number of sideband events have been logged into the ring
buffer. This is problematic if we are only interested in, say, context
switch events. This patch provides for a way to achieve this.

As noted, this is a RFC and I am not too specific about the interface or
the ioctl name. Kindly suggest if you think there is a better way to
achieve this.

- Naveen

---
Here is a sample program demonstrating the same:

    #define _GNU_SOURCE

    #include <stdlib.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <string.h>
    #include <signal.h>
    #include <sys/ioctl.h>
    #include <sys/mman.h>
    #include <linux/perf_event.h>
    #include <asm/unistd.h>

    static long
    perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
		   int cpu, int group_fd, unsigned long flags)
    {
	return syscall(__NR_perf_event_open, hw_event, pid, cpu,
		      group_fd, flags);
    }

    static void sigio_handler(int n, siginfo_t *info, void *uc)
    {
	fprintf (stderr, "Caught %s\n", info->si_code == POLL_HUP ? "POLL_HUP" :
		       (info->si_code == POLL_IN ? "POLL_IN" : "other signal"));

	if (ioctl(info->si_fd, PERF_EVENT_IOC_REFRESH, 2) == -1)
	    perror("SIGIO: IOC_REFRESH");
    }

    int main(int argc, char **argv)
    {
	struct perf_event_attr pe;
	struct sigaction act;
	int fd;
	void *buf;

	memset(&act, 0, sizeof(act));
	act.sa_sigaction = sigio_handler;
	act.sa_flags = SA_SIGINFO;
	sigaction(SIGIO, &act, 0);

	memset(&pe, 0, sizeof(struct perf_event_attr));
	pe.size = sizeof(struct perf_event_attr);
	pe.type = PERF_TYPE_SOFTWARE;
	pe.config = PERF_COUNT_SW_DUMMY;
	pe.disabled = 1;
	pe.sample_period = 1;
	pe.context_switch = 1;

	fd = perf_event_open(&pe, 0, -1, -1, 0);
	if (fd == -1) {
	    fprintf(stderr, "Error opening leader %lx\n", pe.config);
	    exit(EXIT_FAILURE);
	}

	buf = mmap(NULL, sysconf(_SC_PAGESIZE) * 2, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
	if (buf == MAP_FAILED) {
	    fprintf(stderr, "Can't mmap buffer\n");
	    return -1;
	}

	if (fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, 0) | O_ASYNC) == -1)
	    return -2;

	if (fcntl(fd, F_SETSIG, SIGIO) == -1)
	    return -3;

	if (fcntl(fd, F_SETOWN, getpid()) == -1)
	    return -4;

	if (ioctl(fd, PERF_EVENT_IOC_COUNT_RECORDS, 0) == -1)
	    return -5;

	if (ioctl(fd, PERF_EVENT_IOC_REFRESH, 2) == -1)
	    return -6;

	fprintf (stderr, "Sleep 1\n");
	sleep(1);
	fprintf (stderr, "Sleep 2\n");
	sleep(1);
	fprintf (stderr, "Sleep 3\n");
	sleep(1);

	/* Disable the event counter */
	ioctl(fd, PERF_EVENT_IOC_DISABLE, 1);

	close(fd);

	return 0;
    }

A sample output:
    $ time ./cs
    Sleep 1
    Caught POLL_HUP
    Sleep 2
    Caught POLL_HUP
    Sleep 3
    Caught POLL_HUP

    real	0m3.060s
    user	0m0.001s
    sys	0m0.005s


Naveen N. Rao (1):
  kernel/events: Introduce IOC_COUNT_RECORDS

 include/linux/perf_event.h      |  1 +
 include/uapi/linux/perf_event.h |  1 +
 kernel/events/core.c            | 16 +++++++++++++++-
 kernel/events/ring_buffer.c     |  9 +++++++++
 4 files changed, 26 insertions(+), 1 deletion(-)

-- 
2.12.2