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.
> I find this code _really_ hard to read...
Hope the explanation above clarifies the logic... do you have
alternative proposals?
> > @@ -743,6 +929,7 @@ static inline void enqueue_task(struct rq *rq, struct
> > task_struct *p, int flags)
> > if (!(flags & ENQUEUE_RESTORE))
> > sched_info_queued(rq, p);
> >
> > + uclamp_task_update(rq, p);
> > p->sched_class->enqueue_task(rq, p, flags);
> > }
> >
> > @@ -754,6 +941,7 @@ static inline void dequeue_task(struct rq *rq, struct
> > task_struct *p, int flags)
> > if (!(flags & DEQUEUE_SAVE))
> > sched_info_dequeued(rq, p);
> >
> > + uclamp_task_update(rq, p);
> > p->sched_class->dequeue_task(rq, p, flags);
> > }
> >
--
#include <best/regards.h>
Patrick Bellasi
