linux-kernel - Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

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Message-ID: <20180509092823.sfph4gomnblb7jgr@vireshk-i7>
Date:   Wed, 9 May 2018 14:58:23 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Joel Fernandes <joelaf@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency
 requests

On 09-05-18, 02:02, Joel Fernandes wrote:
> On Wed, May 09, 2018 at 02:10:01PM +0530, Viresh Kumar wrote:
> > Right, none of the above changes are required now.
> 
> I didn't follow what you mean the changes are not required? I was developing
> against Linus mainline. Also I replied to Rafael's comment in the other
> thread.

At least for the shared policy case the entire sequence of
sugov_should_update_freq() followed by sugov_update_commit() is
executed from within spinlock protected region and you are using the
same lock below. And so either the above two routines or the kthread
routine below will execute at a given point of time.

So in case kthread has started doing the update and acquired the lock,
the util update handler will wait until the time work_in_progress is
set to false, that's not a problem we are trying to solve here.

And if kthread hasn't acquired the lock yet and util handler has
started executing sugov_should_update_freq() ....

And ^^^ this is where I understood that your earlier change is
actually required, so that we accumulate the latest updated next_freq
value.

And with all that we wouldn't require a while loop in the kthread
code.
 
> > > > @@ -381,13 +381,23 @@ sugov_update_shared(struct update_util_data *hook, u64 time, unsigned int flags)
> > > >  static void sugov_work(struct kthread_work *work)
> > > >  {
> > > >         struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
> > > > +       unsigned int freq;
> > > > +
> > > > +       /*
> > > > +        * Hold sg_policy->update_lock just enough to handle the case where:
> > > > +        * if sg_policy->next_freq is updated before work_in_progress is set to
> > > > +        * false, we may miss queueing the new update request since
> > > > +        * work_in_progress would appear to be true.
> > > > +        */
> > > > +       raw_spin_lock(&sg_policy->update_lock);
> > > > +       freq = sg_policy->next_freq;
> > > > +       sg_policy->work_in_progress = false;
> > > > +       raw_spin_unlock(&sg_policy->update_lock);
> > 
> > One problem we still have is that sg_policy->update_lock is only used
> > in the shared policy case and not in the single CPU per policy case,
> > so the race isn't solved there yet.
> 
> True.. I can make the single CPU case acquire the update_lock very briefly
> around sugov_update_commit call in sugov_update_single.

Rafael was very clear from the beginning that he wouldn't allow a spin
lock in the un-shared policy case :)

-- 
viresh