Re: [PATCH v5] do_wait: make PIDTYPE_PID case O(1) instead of O(n)

[Jim Newsome <jnewsome@torproject.org>]

On 3/12/21 12:22, Andrew Morton wrote:
> 
> Could we please see some performance testing results to permit us to
> evaluate the value of this change?

Sure. I've been doing some ad-hoc measurements with the code below. It
forks 8k children and then waits for them in reverse order (forcing a
full list traversal each time). I'll need to reboot a couple times to
get apples-to-apples measurements on bare metal, though. I'll plan to
run with NUMCHILDREN = 0 -> 8000, by 100.

Does this look like it'd be sufficient, or is there more you'd like to
see? The current form doesn't use ptrace, but I expect the results to be
similar; (maybe more pronounced when tracing threaded children, since
every thread is in the tracee list instead of just the group leaders).

#define NUMCHILDREN 8000

void fork_and_wait() {
    pid_t children[NUMCHILDREN];
    for (int i = 0; i < NUMCHILDREN; ++i) {
        pid_t forkrv = fork();
        if (forkrv < 0) {
            perror("fork");
            exit(1);
        }
        if (forkrv == 0) {
            // child
            exit(0);
        }
        // parent
        children[i] = forkrv;
    }
    for (int i = 0; i < NUMCHILDREN; ++i) {
        int wstatus;
        if (waitpid(children[NUMCHILDREN - i - 1], &wstatus, 0) < 0) {
            perror("waitpid");
            exit(1);
        }
    }
}