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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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