Re: [PATCH v14 5/6] sched/core: uclamp: Update CPU's refcount on TG's clamp changes

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Sep 02, 2019 at 07:44:40AM +0100, Patrick Bellasi wrote:
> On Fri, Aug 30, 2019 at 09:48:34 +0000, Peter Zijlstra wrote...
> > On Thu, Aug 22, 2019 at 02:28:10PM +0100, Patrick Bellasi wrote:

> >> +	rq = task_rq_lock(p, &rf);
> >
> > Since modifying cgroup parameters is priv only, this should be OK I
> > suppose. Priv can already DoS the system anyway.
> 
> Are you referring to the possibility to DoS the scheduler by keep
> writing cgroup attributes?

Yep.

> Because, in that case I think cgroup attributes could be written also by
> non priv users. It all depends on how they are mounted and permissions
> are set. Isn't it?
> 
> Anyway, I'm not sure we can fix that here... and in principle we could
> have that DoS by setting CPUs affinities, which is user exposed.
> Isn't it?

Only for a single task; by using the cgroup thing we have that in-kernel
iteration of tasks.

The thing I worry about is bouncing rq->lock around the system; but
yeah, I suppose a normal user could achieve something similar with
enough tasks.

> >> +	/*
> >> +	 * Setting the clamp bucket is serialized by task_rq_lock().
> >> +	 * If the task is not yet RUNNABLE and its task_struct is not
> >> +	 * affecting a valid clamp bucket, the next time it's enqueued,
> >> +	 * it will already see the updated clamp bucket value.
> >> +	 */
> >> +	if (!p->uclamp[clamp_id].active)
> >> +		goto done;
> >> +
> >> +	uclamp_rq_dec_id(rq, p, clamp_id);
> >> +	uclamp_rq_inc_id(rq, p, clamp_id);
> >> +
> >> +done:
> >
> > I'm thinking that:
> >
> > 	if (p->uclamp[clamp_id].active) {
> > 		uclamp_rq_dec_id(rq, p, clamp_id);
> > 		uclamp_rq_inc_id(rq, p, clamp_id);
> > 	}
> >
> > was too obvious? ;-)
> 
> Yep, right... I think there was some more code in prev versions but I
> forgot to get rid of that "goto" pattern after some change.

OK, already fixed that.