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    SubjectRe: [patch 3/4] taskstats: Introduce cdata_acct for complete cumulative accounting
    On 11/19, Michael Holzheu wrote:
    > Currently the cumulative time accounting in Linux is not complete.
    > Due to POSIX POSIX.1-2001, the CPU time of processes is not accounted
    > to the cumulative time of the parents, if the parents ignore SIGCHLD
    > or have set SA_NOCLDWAIT. This behaviour has the major drawback that
    > it is not possible to calculate all consumed CPU time of a system by
    > looking at the current tasks. CPU time can be lost.
    > This patch adds a new set of cumulative time counters. We then have two
    > cumulative counter sets:
    > * cdata_wait: Traditional cumulative time used e.g. by getrusage.
    > * cdata_acct: Cumulative time that also includes dead processes with
    > parents that ignore SIGCHLD or have set SA_NOCLDWAIT.
    > cdata_acct will be exported by taskstats.

    Looks correct at first glance. A couple of nits below.

    > TODO:
    > -----
    > With this patch we take the siglock twice. First for the dead task
    > and second for the parent of the dead task. This give the following
    > lockdep warning (probably a lockdep annotation is needed here):

    And we already discussed this ;) We do not need 2 siglock's, only
    parent's. Just move the callsite in __exit_signal() down, under
    another (lockless) group_dead check.

    Or I missed something?

    > @@ -595,6 +595,8 @@ struct signal_struct {
    > */
    > struct cdata cdata_wait;
    > struct cdata cdata_threads;
    > + struct cdata cdata_acct;
    > + struct task_io_accounting ioac_acct;
    > struct task_io_accounting ioac;

    Given that task_io_accounting is Linux specific, perhaps we can use
    signal->ioac in both cases?

    Yes, this is a user-visible change anyway. But, at least we can
    forget about POSIX.

    > + spin_lock_irqsave(&p->real_parent->sighand->siglock, flags);
    > + if (wait) {
    > + pcd = &p->real_parent->signal->cdata_wait;
    > + tcd = &p->signal->cdata_threads;
    > + cd = &p->signal->cdata_wait;
    > + } else {
    > + pcd = &p->real_parent->signal->cdata_acct;
    > + tcd = &p->signal->cdata_threads;
    > + cd = &p->signal->cdata_acct;
    > + }

    We can do this before taking ->siglock. Not that I think this really
    matters, but otherwise this looks a bit confusing imho, as if we need
    parent's ->siglock to pin something.

    And thanks for splitting these changes. It was much, much easier to
    read now.


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