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SubjectRe: [RFC/RFT PATCH] sched: automated per tty task groups
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On Thu, 2010-10-21 at 18:29 +0200, Oleg Nesterov wrote:
> On 10/21, Peter Zijlstra wrote:
> >
> > On Thu, 2010-10-21 at 06:51 -0400, Mathieu Desnoyers wrote:
> > > * Mike Galbraith (efault@gmx.de) wrote:
> > > [...]
> > > > +static void
> > > > +autogroup_attach_tty(struct task_struct *p, struct task_group **tg)
> > > > +{
> > > > + struct tty_struct *tty = p->signal->tty;
> > > > +
> > > > + if (!tty)
> > > > + return;
> > > > +
> > > > + *tg = p->signal->tty->tg;
> > > > +}
>
> minor nit, I think in theory this needs barrier(), or
>
> struct tty_struct *tty = ACCESS_ONCE(p->signal->tty);
>
> if (tty)
> *tg = tty->tg;

Thanks.

> > > > +static inline void
> > > > +autogroup_check_attach(struct task_struct *p, struct task_group **tg)
> > > > +{
> > > > + if (!sysctl_sched_autogroup_enabled || *tg != &root_task_group ||
> > > > + p->sched_class != &fair_sched_class)
> > > > + return;
> > > > +
> > > > + rcu_read_lock();
> > > > +
> > > > + autogroup_attach_tty(p, tg);
> > > > +
> > > > + rcu_read_unlock();
> > > > +}
> > > > +
> >
> > > Meanwhile, a little question about locking here: how is
> > > the read lock supposed to protect from p->signal (and p->signal->tty)
> > > modifications ? What's the locking scheme here ? So maybe just simple
> > > rcu_dereference are missing, or maybe the tsk->sighand->siglock might be
> > > required. In all cases, I feel something is missing there.
> >
> > Oleg, could you comment?
>
> No, I don't understand this ;) But I know nothig about task groups,
> most probably this is OK.
>
> It is not clear to me why do we need rcu_read_lock() and how it can help.
> The tty can go away right after dereferencing signal->tty.

It was inherited.

> Even if the task doesn't exit, it (or its sub-thread) can do sys_setsid()
> at any moment and free this tty. If any thread was moved to tty->sg, doesn't
> this mean that, say, ->cfs_rq will point to the already freed tg->cfs_rq?

Ah, so isn't as safe as it looked. Thanks!

> >From http://marc.info/?l=linux-kernel&m=128764874422614
>
> +int sched_autogroup_handler(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> + struct task_struct *p, *t;
> + struct task_group *tg;
> + int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +
> + if (ret || !write)
> + return ret;
> +
> + for_each_process(p) {
>
> Hmm. This needs rcu lock at least?

(used to be paranoid locking there.. vs required locking)

> + tg = task_group(p);
>
> Why?

A cleanup leftover.

>
> + sched_move_task(p);
> + list_for_each_entry_rcu(t, &p->thread_group, thread_group) {
> + sched_move_task(t);
> + }
> + }
>
> Looks like, you can just do
>
> do_each_thread(p, t) {
> sched_move_task(t);
> } while_each_thread(p, t);
>
> With the same effect.

Yeah.

So in theory, the tty can go away on me. I knew this was too easy.

-Mike




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