Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting

[Peter Zijlstra]

On Fri, Apr 13, 2018 at 12:15:10PM +0100, Patrick Bellasi wrote:
> On 13-Apr 10:43, Peter Zijlstra wrote:
> > On Mon, Apr 09, 2018 at 05:56:09PM +0100, Patrick Bellasi wrote:
> > > +static inline void uclamp_task_update(struct rq *rq, struct task_struct *p)
> > > +{
> > > +	int cpu = cpu_of(rq);
> > > +	int clamp_id;
> > > +
> > > +	/* The idle task does not affect CPU's clamps */
> > > +	if (unlikely(p->sched_class == &idle_sched_class))
> > > +		return;
> > > +	/* DEADLINE tasks do not affect CPU's clamps */
> > > +	if (unlikely(p->sched_class == &dl_sched_class))
> > > +		return;
> > > +
> > > +	for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
> > > +		if (uclamp_task_affects(p, clamp_id))
> > > +			uclamp_cpu_put(p, cpu, clamp_id);
> > > +		else
> > > +			uclamp_cpu_get(p, cpu, clamp_id);
> > > +	}
> > > +}
> > 
> > Is that uclamp_task_affects() thing there to fix up the fact you failed
> > to propagate the calling context (enqueue/dequeue) ?
> 
> Not really, it's intended by design: we back annotate the clamp_group
> a task has been refcounted in.
> 
> The uclamp_task_affects() tells if we are refcounted now and then we
> know from the back-annotation from which refcounter we need to remove
> the task.
> 
> I found this solution much less racy and effective in avoiding to
> screw up the refcounter whenever we look at a task at either
> dequeue/migration time and these operations can overlaps with the
> slow-path. Meaning, when we change the task specific clamp_group
> either via syscall or cgroups attributes.
> 
> IOW, the back annotation allows to decouple refcounting from
> clamp_group configuration in a lockless way.

But it adds extra state and logic, to a fastpath, for no reason.

I suspect you messed up the cgroup side; because the syscall should
already have done task_rq_lock() and hold both p->pi_lock and rq->lock
and have dequeued the task when changing the attribute.

It is actually really hard to make the syscall do it wrong.