I didn't read the whole patch, but some parts doesn't look right,
On 10/06, Michael Holzheu wrote:
>
> The patch also approaches another ugly Unix behavior regarding process
> accounting. If a parent process dies before his children, the children
> get the reaper process (init) as new parent. If we want to determine the
> CPU usage of a process tree with cumulative time, this is very
> suboptimal. To fix this I added a new process relationship tree for
> accounting.
Well, I must admit, I can't say I like the complications this change adds ;)
In any case, imho this change needs a separate patch/discussion.
> Besides of that the patch adds an "acct_parent" pointer next to the parent
> pointer and a "children_acct" list next to the children list to the
> task_struct in order to remember the correct accounting task relationship.
I am not sure I understand the "correct accounting" above. ->acct_parent
adds the "parallel" hierarchy. In the simplest case, suppose that some
process P forks the child C and exits. Then C->acct_parent == P->real_parent
(P->acct_parent in general). I am not sure this is always good.
Anyway,
> @@ -90,6 +156,24 @@ static void __exit_signal(struct task_st
>
> posix_cpu_timers_exit(tsk);
> if (group_dead) {
> + if (!tsk->exit_accounting_done) {
> +#ifdef __s390x__
> + /*
> + * FIXME: On s390 we can call account_process_tick to update
> + * CPU time information. This is probably not valid on other
> + * architectures.
> + */
> + if (current == tsk)
> + account_process_tick(current, 1);
> +#endif
> + /*
> + * FIXME: This somehow has to be moved to
> + * finish_task_switch(), because otherwise
> + * if the process accounts itself, the CPU time
> + * that is used for this code will be lost.
> + */
> + __account_to_parent(tsk, 0);
We hold the wrong ->siglock here.
Also, the logic behind ->exit_accounting_done looks wrong (and unneeded)
but I am not sure...
> @@ -772,6 +869,15 @@ static void forget_original_parent(struc
> LIST_HEAD(dead_children);
>
> write_lock_irq(&tasklist_lock);
> + list_for_each_entry_safe(p, n, &father->children_acct, sibling_acct) {
> + struct task_struct *t = p;
> + do {
> + t->acct_parent = t->acct_parent->acct_parent;
> + } while_each_thread(p, t);
> + list_move_tail(&p->sibling_acct,
> + &p->acct_parent->children_acct);
This is certainly wrong if there are other live threads in father's
thread-group.
Also, you need to change de_thread() if it changes the leader.
> list_for_each_entry_safe(p, n, &dead_children, sibling) {
> list_del_init(&p->sibling);
> + list_del_init(&p->sibling_acct);
This list_del() can race with ->acct_parent if it in turn exits and
does forget_original_parent() -> list_move_tail(sibling_acct).
Oleg.
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