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    SubjectRe: [RFC/RFT PATCH v3] sched: automated per tty task groups
    On 11/15, Mike Galbraith wrote:
    >
    > On Mon, 2010-11-15 at 13:57 +0100, Oleg Nesterov wrote:
    >
    > > And the exiting task can do a lot before it disappears, probably
    > > we shouldn't ignore ->autogroup.

    I don't really understand what makes the exiting task different,
    but OK.

    However, I must admit I dislike this check. Because, looking at this
    code, it is not clear why do we check PF_EXITING. It looks as if it
    is needed for correctness.

    OK, this is minor. I think the patch is correct, just one nit below.

    > > It can't protect the change of signal->autogroup, multiple callers
    > > can use different rq's.
    >
    > Guaranteed live ->autogroup should be good enough for heuristic use, and
    > had better be so. Having to take ->siglock in the fast path would kill
    > using ->signal.

    Yes, sure, rq->lock should ensure signal->autogroup can't go away.
    (even if it can be changed under us). And it does, we are moving all
    threads before kref_put().

    > +static void
    > +autogroup_move_group(struct task_struct *p, struct autogroup *ag)
    > +{
    > + struct autogroup *prev;
    > + struct task_struct *t;
    > +
    > + prev = p->signal->autogroup;
    > + if (prev == ag)
    > + return;
    > +
    > + p->signal->autogroup = autogroup_kref_get(ag);
    > + sched_move_task(p);
    > +
    > + rcu_read_lock();
    > + list_for_each_entry_rcu(t, &p->thread_group, thread_group) {
    > + sched_move_task(t);
    > + }
    > + rcu_read_unlock();
    > +
    > + autogroup_kref_put(prev);
    > +}

    Well, this looks a bit strange (but correct).

    We are changing ->autogroup assuming the caller holds ->siglock.
    But if we hold ->siglock we do not need rcu_read_lock() to iterate
    over the thread_group, we can just do

    p->signal->autogroup = autogroup_kref_get(ag);

    t = p;
    do {
    sched_move_task(t);
    } while_each_thread(p, t);

    Again, this is minor, I won't insist.

    Oleg.



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