Re: [PATCH v6 05/16] sched/core: uclamp: Update CPU's refcount on clamp changes

Patrick Bellasi
2019-01-22T14:01:15Z

On 22-Jan 14:28, Peter Zijlstra wrote:
> On Tue, Jan 22, 2019 at 10:43:05AM +0000, Patrick Bellasi wrote:
> > On 22-Jan 10:37, Peter Zijlstra wrote:
>
> > > Sure, I get that. What I don't get is why you're adding that (2) here.
> > > Like said, __sched_setscheduler() already does a dequeue/enqueue under
> > > rq->lock, which should already take care of that.
> >
> > Oh, ok... got it what you mean now.
> >
> > With:
> >
> > [PATCH v6 01/16] sched/core: Allow sched_setattr() to use the current policy
> > <20190115101513.2822-2-patrick.bellasi@arm.com>
> >
> > we can call __sched_setscheduler() with:
> >
> > attr->sched_flags & SCHED_FLAG_KEEP_POLICY
> >
> > whenever we want just to change the clamp values of a task without
> > changing its class. Thus, we can end up returning from
> > __sched_setscheduler() without doing an actual dequeue/enqueue.
>
> I don't see that happening.. when KEEP_POLICY we set attr.sched_policy =
> SETPARAM_POLICY. That is then checked again in __setscheduler_param(),
> which is in the middle of that dequeue/enqueue.

Yes, I think I've forgot a check before we actually dequeue the task.

The current code does:

---8<---
SYSCALL_DEFINE3(sched_setattr)

// A) request to keep the same policy
if (attr.sched_flags & SCHED_FLAG_KEEP_POLICY)
attr.sched_policy = SETPARAM_POLICY;

sched_setattr()
// B) actually enforce the same policy
if (policy < 0)
policy = oldpolicy = p->policy;

// C) tune the clamp values
if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
retval = __setscheduler_uclamp(p, attr);

// D) tune attributes if policy is the same
if (unlikely(policy == p->policy))
if (fair_policy(policy) && attr->sched_nice != task_nice(p))
goto change;
if (rt_policy(policy) && attr->sched_priority != p->rt_priority)
goto change;
if (dl_policy(policy) && dl_param_changed(p, attr))
goto change;
return 0;
change:

// E) dequeue/enqueue task
---8<---

So, probably in D) I've missed a check on SCHED_FLAG_KEEP_POLICY to
enforce a return in that case...

> Also, and this might be 'broken', SETPARAM_POLICY _does_ reset all the
> other attributes, it only preserves policy, but it will (re)set nice
> level for example (see that same function).

Mmm... right... my bad! :/

> So maybe we want to introduce another (few?) FLAG_KEEP flag(s) that
> preserve the other bits; I'm thinking at least KEEP_PARAM and KEEP_UTIL
> or something.

Yes, I would say we have two options:

1) SCHED_FLAG_KEEP_POLICY enforces all the scheduling class specific
attributes, but cross class attributes (e.g. uclamp)

2) add SCHED_KEEP_NICE, SCHED_KEEP_PRIO, and SCED_KEEP_PARAMS
and use them in the if conditions in D)

In both cases the goal should be to return from code block D).

What do you prefer?

--
#include <best/regards.h>

Patrick Bellasi