Messages in this thread |  | | Date | Fri, 13 Apr 2018 13:36:50 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting |
| |
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.
|  |