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

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

quoted

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

quoted

quoted

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;
                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