[RFC PATCH v2 0/2] Notifications for perf sideband events

[Naveen N. Rao]

v2:
Here is a slightly different approach compared to v1: instead of
introducing a new IOCTL, we ride on top of the existing support for
ring buffer wakeup to generate signals as desired. Counting sideband
events still requires some changes in the output path, but in normal
cases, it ends up being just a comparison.

The test program below needs to be changed for this version. Add below
and remove the ioctl call for PERF_EVENT_IOC_COUNT_RECORDS:
	pe.signal_on_wakeup = 1;
	pe.count_sb_events = 1;
	pe.wakeup_events = 2;

Also, I have not handled PERF_EVENT_IOC_REFRESH in this version, so
IOC_ENABLE needs to be used.

v1:
https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1414197.html

Cover letter from v1:
--------------------

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 (2):
  kernel/events: Add option to notify through signals on wakeup
  kernel/events: Add option to enable counting sideband events in
    wakeup_events

 include/uapi/linux/perf_event.h |  4 +++-
 kernel/events/core.c            | 14 ++++++++------
 kernel/events/ring_buffer.c     | 16 ++++++++++++++++
 3 files changed, 27 insertions(+), 7 deletions(-)

-- 
2.12.2