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

From: Naveen N. Rao
Date: Tue Aug 01 2017 - 10:44:59 EST

- Patch 1: Some cosmetic updates to address Peter's feedback
- Patch 2: Add unlikely() hint for test in __perf_output_begin() so as
to minimize impact in normal cases.
- Patch 2: Factor out handling of wakeup_events into a separate helper.

This series has also been tested with perf_event_tests testsuite to
ensure there are no regressions.

- Naveen


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. Furthermore, signals are more useful (rather than polling) for
self-profiling. This series provides for a way to achieve this.

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 demonstrates how a process can profile itself for
context switch events and how it can control notification through
signals. The key changes include the below perf_event_attr settings as
well as use of IOC_ENABLE:
pe.signal_on_wakeup = 1;
pe.count_sb_events = 1;
pe.wakeup_events = 2;

To keep things simple, PERF_EVENT_IOC_REFRESH cannot be used if any of
the new attributes are set.

- 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"));

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.config = PERF_COUNT_SW_DUMMY;
pe.disabled = 1;
pe.sample_period = 1;
pe.context_switch = 1;
pe.signal_on_wakeup = 1;
pe.count_sb_events = 1;
pe.wakeup_events = 2;

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

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_ENABLE, 0) == -1)
return -5;

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

/* Disable the event counter */


return 0;

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

real 0m3.040s
user 0m0.001s
sys 0m0.003s

Naveen N. Rao (2):
kernel/events: Add option to notify through signals on wakeup
kernel/events: Add option to enable counting sideband events in

include/uapi/linux/perf_event.h | 4 +++-
kernel/events/core.c | 39 ++++++++++++++++++---------------------
kernel/events/internal.h | 1 +
kernel/events/ring_buffer.c | 21 +++++++++++++++++++++
4 files changed, 43 insertions(+), 22 deletions(-)