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

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 22-Jan 15:57, Peter Zijlstra wrote:
> On Tue, Jan 22, 2019 at 02:01:15PM +0000, Patrick Bellasi wrote:
> > 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;
> 
> 		  if (util_changed)
> 		      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)
> 
> So the current KEEP_POLICY basically provides sched_setparam(), and

But it's not exposed user-space.

> given we have that as a syscall, that is supposedly a useful
> functionality.

For uclamp is definitively useful to change clamps without the need to
read beforehand the current policy params and use them in a following
set syscall... which is racy pattern.

> Also, NICE/PRIO/DL* is all the same thing and depends on the policy,
> KEEP_PARAM should cover the lot

Right, that makes sense.

> And I suppose the UTIL_CLAMP is !KEEP_UTIL; we could go either way
> around with that flag.

What about getting rid of the racy case above by exposing userspace
only the new UTIL_CLAMP and, on:

  sched_setscheduler(flags: UTIL_CLAMP)

we enforce the other two flags from the syscall:

---8<---
        SYSCALL_DEFINE3(sched_setattr)
            if (attr.sched_flags & SCHED_FLAG_KEEP_POLICY) {
                attr.sched_policy = SETPARAM_POLICY;
                attr.sched_flags |= (KEEP_POLICY|KEEP_PARAMS);
            }
---8<---

This will not make possible to change class and set flags in one go,
but honestly that's likely a very limited use-case, isn't it ?

> > In both cases the goal should be to return from code block D).
> 
> I don't think so; we really do want to 'goto change' for util changes
> too I think. Why duplicate part of that logic?

But that will force a dequeue/enqueue... isn't too much overhead just
to change a clamp value? Perhaps we can also end up with some wired
side-effects like the task being preempted ?

Consider also that the uclamp_task_update_active() added by this patch
not only has lower overhead but it will be use also by cgroups where
we want to force update all the tasks on a cgroup's clamp change.

-- 
#include <best/regards.h>

Patrick Bellasi