[PATCH 0/2] perf: Add PERF_EVENT_IOC_SET_EVENT_LIMIT

[Charlie Jenkins]

My motivation here is to have a program that is able to detect when an
arbitrary other program has reviewed some number of PMU overflows.
PERF_EVENT_IOC_REFRESH is not useful in the scenario since that ioctl
enables the event immediately. This new ioctl flag
PERF_EVENT_IOC_SET_EVENT_LIMIT allows a program to setup a event_limit
and then enable the event when ready at some later time.

This solves the first problem. The second problem that I am debating a
solution for is that the refresh SIGIO signals are marked as duplicate
signals and not always delivered to userspace. I will attach a simple
program below that can be ran a handful of times to catch this behavior.
REFRESH is supposed to send a SIGIO for every overflow, with an si_code
of POLL_IN for each signal except the last one which has a si_code of
POLL_HUP. However, __send_signal_locked() in kernel/signal.c will
occasionally catch this POLL_HUP signal before all POLL_IN signals have
been processed, thus treating this signal as a duplicate and dropping
with this logic:

	pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;
	/*
	 * Short-circuit ignored signals and support queuing
	 * exactly one non-rt signal, so that we can get more
	 * detailed information about the cause of the signal.
	 */
	result = TRACE_SIGNAL_ALREADY_PENDING;
	if (legacy_queue(pending, sig))
		goto ret;

I am sending this patch to begin a conversation on this issue with
REFRESH. One solution could be to only treat a signal as a duplicate in
__send_signal_locked() if the si_code is the same. Another solution is
to only use event_limits with a value of one, as any other number will
run into this race condition. If consistent signals are desired, sigtrap
can be used, but that also doesn't support enable_on_exec.

Here is a minimal test program for dropped signals. Can be ran on a loop until
program halts waiting for signal that will never arrive.

while ./signal 1 1; do sleep .01; done

// signal.c

int fd;
pid_t pid;

struct read_format {
	unsigned long long value;
	unsigned long long time_enabled;
	unsigned long long time_running;
};

int counter;

static inline int sys_perf_event_open(struct perf_event_attr *attr, int pid, int cpu, int group_fd,
		        	      unsigned long flags)
{
	return syscall(__NR_perf_event_open, attr, pid, cpu, group_fd, flags);
}

static void sig_handler(int signum, siginfo_t *siginfo, void *user_context)
{
	struct read_format rd;
	counter++;
	read(fd, &rd, sizeof(rd));
	printf("fd: %d, signum: %d, si_code: %d, counter: %llu, time to overflow: %llu. time enabled: %llu\n", signum, siginfo->si_fd, siginfo->si_code, rd.value, rd.time_running, rd.time_enabled);
	if (counter == REFRESH_COUNT) {
		printf("Hit refresh count\n");
		close(fd);
		kill(pid, SIGKILL);
		if (siginfo->si_code == POLL_HUP) {
			exit(EXIT_SUCCESS); 
		} else {
			printf("INCORRECT COUNT! Expected POLL_HUP on second signal.\n");
			exit(EXIT_FAILURE); 
		}
	}
}

int main(int argc, char *argv[])
{
	char sbuf[128];

	if (argc < 3)
		goto invalid_args;

	unsigned long long num_instructions = atoll(argv[1]);
	unsigned int num_samples = atoi(argv[2]);
	unsigned long long sample_periods = num_instructions / num_samples;

	pid = fork();

	if (pid) {
		struct sigaction sa = {
			.sa_sigaction = (void *) sig_handler,
			.sa_flags = SA_SIGINFO
		};
		struct perf_event_attr pe = {
			.type = PERF_TYPE_HARDWARE,
			.size = sizeof(pe),
			.config = PERF_COUNT_HW_INSTRUCTIONS,
			.sample_period = sample_periods,
			.wakeup_events = 1,
			.disabled = 1,
			.sample_type = PERF_SAMPLE_IP | PERF_SAMPLE_TID | PERF_SAMPLE_CPU | PERF_SAMPLE_PERIOD,
			.read_format = PERF_FORMAT_TOTAL_TIME_RUNNING | PERF_FORMAT_TOTAL_TIME_ENABLED,
			.exclude_kernel = 1,
			.exclude_idle = 1,
			.exclude_hv = 1
		};

		if (sigaction(SIGIO, &sa, NULL) < 0) {
			fprintf(stderr, "FAILED setting up signal handler\n");
			exit(EXIT_FAILURE);
		}

		fd = sys_perf_event_open(&pe, pid, -1, -1, PERF_FLAG_FD_CLOEXEC);

		if (fd == -1) {
			fprintf(stderr, "FAILED opening perf: %s\n", strerror_r(errno, sbuf, sizeof(sbuf)));
			exit(EXIT_FAILURE);
		}

		ioctl(fd, PERF_EVENT_IOC_RESET, 0);
		ioctl(fd, PERF_EVENT_IOC_REFRESH, REFRESH_COUNT);

		fcntl(fd, F_SETFL, O_RDWR | O_NONBLOCK | O_ASYNC);
		fcntl(fd, F_SETSIG, SIGIO);
		fcntl(fd, F_SETOWN, getpid());

		while (1);
	} else {
		while (1);
	}

invalid_args:
	printf("FAILED: Please provide at least 2 args: number of instructions, number of samples. eg: %s 10000 5\n", argv[0]);
	exit(EXIT_FAILURE);
}

Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
---
Charlie Jenkins (2):
      perf: Add PERF_EVENT_IOC_INC_EVENT_LIMIT
      perf: Document PERF_EVENT_IOC_SET_EVENT_LIMIT

 include/linux/perf_event.h            |  4 ++--
 include/uapi/linux/perf_event.h       |  1 +
 kernel/events/core.c                  | 17 +++++++++++------
 tools/include/uapi/linux/perf_event.h |  1 +
 tools/perf/design.txt                 |  5 +++++
 5 files changed, 20 insertions(+), 8 deletions(-)
---
base-commit: 0c3836482481200ead7b416ca80c68a29cfdaabd
change-id: 20240724-perf_set_event_limit-079f1b996376
-- 
- Charlie