Messages in this thread | | | From | Nadav Amit <> | Subject | Using perf_event_open() to sample multiple events of a process | Date | Thu, 4 Nov 2021 22:57:50 -0700 |
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Hello Ken, Peter,
I would appreciate some help regarding the use of perf_event_open() to have multiple samples getting into the same mmap’d memory when they are both attached to the same process.
I am doing so (using both PERF_FLAG_FD_NO_GROUP and PERF_FLAG_FD_OUTPUT), but it results in -EINVAL. Debugging the code shows that perf_event_set_output() fails due to the following check:
/* * If its not a per-cpu rb, it must be the same task. */ if (output_event->cpu == -1 && output_event->ctx != event->ctx) goto out;
However, it appears that at this point, event->ctx is not initialized (it is null) so the test fails and the whole perf_event_open() syscall fails.
Am I missing something? If not, I am unsure, unfortunately, what the proper way to fix it is…
I include a small test that fails on my system. The second perf_event_open fails due to the check in perf_event_set_output():
#define _GNU_SOURCE 1
#include <asm/unistd.h> #include <linux/perf_event.h> #include <linux/hw_breakpoint.h> #include <sys/mman.h> #include <unistd.h> #include <stdlib.h> #include <stdio.h>
long perf_event_open(struct perf_event_attr* event_attr, pid_t pid, int cpu, int group_fd, unsigned long flags) { return syscall(__NR_perf_event_open, event_attr, pid, cpu, group_fd, flags); }
int main(void) { pid_t pid = getpid(); int group_fd, fd; void *p; struct perf_event_attr pe = { .type = 4, .size = sizeof(struct perf_event_attr), .config = 0x11d0, .sample_type = 0x8, .sample_period = 1000, .precise_ip = 2, };
group_fd = perf_event_open(&pe, pid, -1, -1, PERF_FLAG_FD_CLOEXEC | PERF_FLAG_FD_NO_GROUP | PERF_FLAG_FD_OUTPUT);
if (group_fd < 0) { perror("first perf_event_open"); exit(-1); }
p = mmap(NULL, 3 * 4096, PROT_READ|PROT_WRITE, MAP_SHARED, group_fd, 0);
if (p == MAP_FAILED) { perror("MAP_FAILED"); exit(-1); } pe.config = 0x12d0; fd = perf_event_open(&pe, pid, -1, group_fd, PERF_FLAG_FD_CLOEXEC | PERF_FLAG_FD_NO_GROUP | PERF_FLAG_FD_OUTPUT);
if (fd < 0) { perror("second perf_event_open"); exit(-1); }
printf("success\n"); }
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