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Date:   Tue, 22 Jan 2019 14:01:15 +0000
From:   Patrick Bellasi <patrick.bellasi@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        linux-api@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Paul Turner <pjt@google.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Todd Kjos <tkjos@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Steve Muckle <smuckle@google.com>,
        Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v6 05/16] sched/core: uclamp: Update CPU's refcount on
 clamp changes
Message-ID: <20190122140115.twtx646vewm757ca@e110439-lin>
References: <20190115101513.2822-1-patrick.bellasi@arm.com>
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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