Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task cookie on fork

[Joel Fernandes]

On Tue, Dec 01, 2020 at 08:20:50PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 01, 2020 at 02:11:33PM -0500, Joel Fernandes wrote:
> > On Wed, Nov 25, 2020 at 12:15:41PM +0100, Peter Zijlstra wrote:
> > > On Tue, Nov 17, 2020 at 06:19:52PM -0500, Joel Fernandes (Google) wrote:
> > > 
> > > > +/*
> > > > + * Ensure that the task has been requeued. The stopper ensures that the task cannot
> > > > + * be migrated to a different CPU while its core scheduler queue state is being updated.
> > > > + * It also makes sure to requeue a task if it was running actively on another CPU.
> > > > + */
> > > > +static int sched_core_task_join_stopper(void *data)
> > > > +{
> > > > +	struct sched_core_task_write_tag *tag = (struct sched_core_task_write_tag *)data;
> > > > +	int i;
> > > > +
> > > > +	for (i = 0; i < 2; i++)
> > > > +		sched_core_tag_requeue(tag->tasks[i], tag->cookies[i], false /* !group */);
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
> > > > +{
> > > 
> > > > +	stop_machine(sched_core_task_join_stopper, (void *)&wr, NULL);
> > > 
> > > > +}
> > > 
> > > This is *REALLY* terrible...
> > 
> > I pulled this bit from your original patch. Are you concerned about the
> > stop_machine? Sharing a core is a slow path for our usecases (and as far as I
> > know, for everyone else's). We can probably do something different if that
> > requirement changes.
> > 
> 
> Yeah.. so I can (and was planning on) remove stop_machine() from
> sched_core_{dis,en}able() before merging it.
> 
> (there's two options, one uses stop_cpus() with the SMT mask, the other
> RCU)

Ok. What about changing the cookie of task T while holding the rq/pi locks, and
then doing a resched_curr(rq) for that RQ?

The holding lock ensures no migration of task happens, and resched_curr()
ensure that task T's rq will enter the scheduler to consider the task T's new
cookie for scheduling. I believe this is analogous to what
__sched_setscheduler() does when you switch a task from CFS to RT.

> This though is exposing stop_machine() to joe user. Everybody is allowed
> to prctl() it's own task and set a cookie on himself. This means you
> just made giant unpriv DoS vector.
> 
> stop_machine is bad, really bad.

Agreed.

thanks,

 - Joel