On Thu, 2009-08-20 at 15:49 +0200, stephane eranian wrote:
> Hi,
>
> I am running into an issue trying to use enable_on_exec
> in per-thread mode with an event group.
>
> My understanding is that enable_on_exec allows activation
> of an event on first exec. This is useful for tools monitoring
> other tasks and which you invoke as: tool my_program. In
> other words, the tool forks+execs my_program. This option
> allows developers to setup the events after the fork (to get
> the pid) but before the exec(). Only execution after the exec
> is monitored. This alleviates the need to use the
> ptrace(PTRACE_TRACEME) call.
>
> My understanding is that an event group is scheduled only
> if all events in the group are active (disabled=0). Thus, one
> trick to activate a group with a single ioctl(PERF_IOC_ENABLE)
> is to enable all events in the group except the leader. This works
> well. But once you add enable_on_exec on on the events,
> things go wrong. The non-leader events start counting before
> the exec. If the non-leader events are created in disabled state,
> then they never activate on exec.
>
> The attached test program demonstrates the problem.
> simply invoke with a program that runs for a few seconds.
OK, lots of issues here
1) your code is broken ;-)
2) enable_on_exec on !leader counters is undefined
3) there is something fishy non the less
1. you fork() then create a counter group in both the parent and the
child without sync, then read the parent group. This obviously doesn't
do what is expected. See attached proglet for a better version.
2. enable_on_exec only works on leaders, Paul, was that intended?
3. the scale stuff seems broken
# perf stat -e cycles -e instructions --repeat 10 true
Performance counter stats for 'true' (10 runs):
2612124 cycles ( +- 1.327% )
1870479 instructions # 0.716 IPC ( +- 0.132% )
0.003743155 seconds time elapsed ( +- 1.203% )
# ./test-enable_on_exec true
2651600 PERF_COUNT_HW_CPU_CYCLES 1111509 1111509 2651600.000000
1832720 PERF_COUNT_HW_INSTRUCTIONS 839395242 1111509
1384043177.264637
Paul, would a counter's time start running when its 'enabled' but part
of a non-runnable group?
---
#include "perf.h"
#include <sys/types.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <syscall.h>
#include <err.h>
#include <fcntl.h>
int child(char **arg)
{
int i;
/* burn cycles to detect if monitoring start before exec */
for(i=0; i < 5000000; i++) syscall(__NR_getpid);
execvp(arg[0], arg);
errx(1, "cannot exec: %s\n", arg[0]);
/* not reached */
}
int parent(char **arg)
{
struct perf_counter_attr hw[2];
char *name[2];
int fd[2];
int status, ret, i;
uint64_t values[3];
pid_t pid;
int child_ready_pipe[2], go_pipe[2];
char buf;
if (pipe(child_ready_pipe) < 0 || pipe(go_pipe) < 0) {
perror("Failed to create pipes");
exit(1);
}
if ((pid=fork()) == -1)
err(1, "Cannot fork process");
if (pid == 0) {
close(child_ready_pipe[0]);
close(go_pipe[1]);
fcntl(go_pipe[0], F_SETFD, FD_CLOEXEC);
execvp("", (char **)arg);
/*
* let the parent know we exist
*/
close(child_ready_pipe[1]);
/*
* wait for the parent to attach its counters
*/
if (read(go_pipe[0], &buf, 1) == -1)
perror("unable to read go_pipe");
exit(child(arg));
}
close(child_ready_pipe[1]);
close(go_pipe[0]);
/*
* wait for the child to appear
*/
if (read(child_ready_pipe[0], &buf, 1) == -1)
perror("unable to read child_ready_pipe");
close(child_ready_pipe[0]);
memset(hw, 0, sizeof(hw));
name[0] = "PERF_COUNT_HW_CPU_CYCLES";
hw[0].type = PERF_TYPE_HARDWARE;
hw[0].config = PERF_COUNT_HW_CPU_CYCLES;
hw[0].read_format =
PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING;
hw[0].disabled = 1;
hw[0].enable_on_exec = 1;
name[1] = "PERF_COUNT_HW_INSTRUCTIONS";
hw[1].type = PERF_TYPE_HARDWARE;
hw[1].config = PERF_COUNT_HW_INSTRUCTIONS;
hw[1].read_format =
PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING;
hw[1].disabled = 0;
hw[1].enable_on_exec = 0;
fd[0] = sys_perf_counter_open(&hw[0], pid, -1, -1, 0);
if (fd[0] == -1)
err(1, "cannot open event0");
fd[1] = sys_perf_counter_open(&hw[1], pid, -1, fd[0], 0);
if (fd[1] == -1)
err(1, "cannot open event1");
/*
* we're good to go, let the child rip
*/
close(go_pipe[1]);
waitpid(pid, &status, 0);
for(i=0; i < 2; i++) {
ret = read(fd[i], values, sizeof(values));
if (ret < sizeof(values))
err(1, "cannot read values event %s", name[i]);
printf("%20"PRIu64" %s %ld %ld %f\n",
values[0],
name[i],
values[1], values[2],
values[2] ? (double)values[0] * values[1]/values[2] :
0);
close(fd[i]);
}
return 0;
}
int main(int argc, char **argv)
{
if (!argv[1])
errx(1, "you must specify a command to execute\n");
return parent(argv+1);
}
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