On 22-Jan 15:57, Peter Zijlstra wrote:

On Tue, Jan 22, 2019 at 02:01:15PM +0000, Patrick Bellasi wrote:

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On 22-Jan 14:28, Peter Zijlstra wrote:

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On Tue, Jan 22, 2019 at 10:43:05AM +0000, Patrick Bellasi wrote:

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On 22-Jan 10:37, Peter Zijlstra wrote:

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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
   [ref]

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;

?

quoted

                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...

quoted

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! :/

quoted

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 ?

quoted

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