exit: reparent: avoid find_new_reaper() if no children

Now that pid_ns logic was isolated we can change forget_original_parent()
to return right after find_child_reaper() when father->children is empty,
there is nothing to reparent in this case.

In particular this avoids find_alive_thread() and this can help if the
whole process exits and it has a lot of PF_EXITING threads at the start of
the thread list, this can easily lead to O(nr_threads ** 2) iterations.

Trivial test case (tested under KVM, 2 CPUs):

    static void *tfunc(void *arg)
    {
        pause();
        return NULL;
    }

    static int child(unsigned int nt)
    {
        pthread_t pt;

        while (nt--)
            assert(pthread_create(&pt, NULL, tfunc, NULL) == 0);

        pthread_kill(pt, SIGTRAP);
        pause();
        return 0;
    }

    int main(int argc, const char *argv[])
    {
        int stat;
        unsigned int nf = atoi(argv[1]);
        unsigned int nt = atoi(argv[2]);

        while (nf--) {
            if (!fork())
                return child(nt);

            wait(&stat);
            assert(stat == SIGTRAP);
        }

        return 0;
    }

$ time ./test 16 16536 shows:

              real        user         sys
    -    5m37.628s    0m4.437s    8m5.560s
    +    0m50.032s    0m7.130s    1m4.927s
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Sterling Alexander <stalexan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -571,6 +571,8 @@ static void forget_original_parent(struct task_struct *father)
 
 	/* Can drop and reacquire tasklist_lock */
 	reaper = find_child_reaper(father);
+	if (list_empty(&father->children))
+		goto unlock;
 
 	reaper = find_new_reaper(father, reaper);
 	list_for_each_entry(p, &father->children, sibling) {
@@ -591,6 +593,7 @@ static void forget_original_parent(struct task_struct *father)
 			reparent_leader(father, p, &dead_children);
 	}
 	list_splice_tail_init(&father->children, &reaper->children);
+ unlock:
 	write_unlock_irq(&tasklist_lock);
 
 	list_for_each_entry_safe(p, n, &dead_children, ptrace_entry) {