Notifications for perf sideband events

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/[email protected]/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

Add a new option 'signal_on_wakeup' to request for a signal to be delivered on ring buffer wakeup controlled through watermark and {wakeup_events, wakeup_watermark}.

---
 include/uapi/linux/perf_event.h |  3 ++-
 kernel/events/core.c            | 12 +++++++-----
 kernel/events/ring_buffer.c     |  3 +++
 3 files changed, 12 insertions(+), 6 deletions(-)

index b1c0b187acfe..e5810b1d74a4 100644
@@ -345,7 +345,8 @@ struct perf_event_attr {
 				context_switch :  1, /* context switch data */
 				write_backward :  1, /* Write ring buffer from end to beginning */
 				namespaces     :  1, /* include namespaces data */
-				__reserved_1   : 35;
+				signal_on_wakeup : 1, /* send signal on wakeup */
+				__reserved_1   : 34;
 
 	union {
 		__u32		wakeup_events;	  /* wakeup every n events */

index 6c4e523dc1e2..73ad30e124e5 100644
@@ -7362,12 +7362,14 @@ static int __perf_event_overflow(struct perf_event *event,
 	 * events
 	 */
 
-	event->pending_kill = POLL_IN;
-	if (events && atomic_dec_and_test(&event->event_limit)) {
-		ret = 1;
-		event->pending_kill = POLL_HUP;
+	if (!event->attr.signal_on_wakeup) {
+		event->pending_kill = POLL_IN;
+		if (events && atomic_dec_and_test(&event->event_limit)) {
+			ret = 1;
+			event->pending_kill = POLL_HUP;
 
-		perf_event_disable_inatomic(event);
+			perf_event_disable_inatomic(event);
+		}
 	}
 
 	READ_ONCE(event->overflow_handler)(event, data, regs);

index 2831480c63a2..4e7c728569a8 100644
@@ -21,6 +21,9 @@ static void perf_output_wakeup(struct perf_output_handle *handle)
 {
 	atomic_set(&handle->rb->poll, POLLIN);
 
+	if (handle->event->attr.signal_on_wakeup)
+		handle->event->pending_kill = POLL_IN;
+
 	handle->event->pending_wakeup = 1;
 	irq_work_queue(&handle->event->pending);
 }

2.12.2

On Tue, Jun 13, 2017 at 05:03:42PM +0530, Naveen N. Rao wrote:

SNIP

Hi Jiri,

Yes, I suspect so (though I'm open to changing the name ;) I haven't handled POLL_HUP properly in this patch, including IOC_REFRESH. I will look into that next.

I wanted to know if the overall approach is fine -- between RFC v1 vs. this version.

Thanks for the review!
- Naveen

Many sideband events are interesting by themselves. When profiling only for sideband events, it is useful to be able to control process wakeup (wakeup_events) based on sideband events alone. Add a new option 'count_sb_events' to do the same.

---
 include/uapi/linux/perf_event.h |  3 ++-
 kernel/events/core.c            |  2 +-
 kernel/events/ring_buffer.c     | 13 +++++++++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

index e5810b1d74a4..ab4dc9a02151 100644
@@ -346,7 +346,8 @@ struct perf_event_attr {
 				write_backward :  1, /* Write ring buffer from end to beginning */
 				namespaces     :  1, /* include namespaces data */
 				signal_on_wakeup : 1, /* send signal on wakeup */
-				__reserved_1   : 34;
+				count_sb_events  : 1, /* wakeup_events also counts sideband events */
+				__reserved_1   : 33;
 
 	union {
 		__u32		wakeup_events;	  /* wakeup every n events */

index 73ad30e124e5..1d8cd3911356 100644
@@ -5955,7 +5955,7 @@ void perf_output_sample(struct perf_output_handle *handle,
 		}
 	}
 
-	if (!event->attr.watermark) {
+	if (!event->attr.count_sb_events && !event->attr.watermark) {
 		int wakeup_events = event->attr.wakeup_events;
 
 		if (wakeup_events) {

index 4e7c728569a8..f43a6081141f 100644
@@ -197,6 +197,19 @@ __perf_output_begin(struct perf_output_handle *handle,
 	 * none of the data stores below can be lifted up by the compiler.
 	 */
 
+	if (event->attr.count_sb_events && !event->attr.watermark) {
+		int wakeup_events = event->attr.wakeup_events;
+
+		if (wakeup_events) {
+			int events = local_inc_return(&rb->events);
+
+			if (events >= wakeup_events) {
+				local_sub(wakeup_events, &rb->events);
+				local_inc(&rb->wakeup);
+			}
+		}
+	}
+
 	if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
 		local_add(rb->watermark, &rb->wakeup);
 

2.12.2