Hi,
> Trying using a program with established results. e.g., This is from
> PeterZ years ago. On intel 'perf stat -e instructions a.out' should show
> ~1 billion (a wee bit more, but clearly in the 1b range).
That program itself gave the expected results, but it's a good
suggestion. When I tried changing that program to also fork a bunch
of child processes, I started getting similarly weird results to what
I had seen before.
Looking more closely at the behavior of perf, I think the essential
difference is that perf uses the *child* PID as argument to
perf_event_open. When I tried changing my program to do the same, it
seems to have fixed the problem. That is to say, where I used
something like
fd = perf_event_open(..., getpid(), ...);
child = fork();
if (child == 0) {
execvp(...);
}
I needed to instead use
pipe(pipefd);
child = fork();
if (child == 0) {
read(pipefd[0], &x, 1);
execvp(...);
}
fd = perf_event_open(..., child, ...);
write(pipefd[1], &x, 1);
This seems like a bug, especially since it only seems to come up when
there are grandchild processes involved. (And I can't help thinking
the first version is a lot more elegant!) Does the kernel somehow get
confused because enable_on_exec is set and the original process hasn't
actually exec'ed anything?
The problem doesn't occur *every* time there are grandchildren - I
didn't see any problems when running some basic experiments with shell
scripts. But here's a simple C program that does exhibit the problem:
int main()
{
int i, j;
for (i = 0; i < 50; i++) {
if (!fork()) {
for (i = 0; i < 100000000; i++) {
asm("nop");
asm("nop");
asm("nop");
asm("nop");
asm("nop");
asm("nop");
asm("nop");
}
exit(0);
}
}
for (i = 0; i < 50; i++)
wait(&j);
return 0;
}
At any rate, thanks for your advice!
Benjamin
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