Re: [PATCH V3 1/3] sched: cpufreq: Allow remote cpufreq callbacks

[Peter Zijlstra]

On Mon, Jul 24, 2017 at 04:31:22PM +0530, Viresh Kumar wrote:
> On 21-07-17, 15:03, Peter Zijlstra wrote:
> > On Thu, Jul 13, 2017 at 12:14:37PM +0530, Viresh Kumar wrote:

> > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > index 29a397067ffa..ed9c589e5386 100644
> > > --- a/kernel/sched/cpufreq_schedutil.c
> > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > @@ -218,6 +218,10 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> > >  	unsigned int next_f;
> > >  	bool busy;
> > >  
> > > +	/* Remote callbacks aren't allowed for policies which aren't shared */
> > > +	if (smp_processor_id() != hook->cpu)
> > > +		return;
> > > +
> > >  	sugov_set_iowait_boost(sg_cpu, time, flags);
> > >  	sg_cpu->last_update = time;
> > >  
> > > @@ -290,6 +294,10 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> > >  	unsigned long util, max;
> > >  	unsigned int next_f;
> > >  
> > > +	/* Don't allow remote callbacks */
> > > +	if (smp_processor_id() != hook->cpu)
> > > +		return;
> > > +
> > >  	sugov_get_util(&util, &max);
> > >  
> > >  	raw_spin_lock(&sg_policy->update_lock);
> > 
> > 
> > Given the whole rq->lock thing, I suspect we could actually not do these
> > two.
> 
> You meant sugov_get_util() and raw_spin_lock()? Why?
> 
> The locking is required here in the shared-policy case to make sure
> only one CPU is updating the frequency for the entire policy. And we
> can't really avoid that even with the rq->lock guarantees from the
> scheduler for the target CPU. 

I said nothing about the shared locking. That is indeed required. All I
said is that those two tests you add could be left out.

> > That would then continue to process the iowait and other accounting
> > stuff, but stall the moment we call into the actual driver, which will
> > then drop the request on the floor as per the first few hunks.
> 
> I am not sure I understood your comment completely though.

Since we call cpufreq_update_util(@rq, ...) with @rq->lock held, all
such calls are in fact serialized for that cpu. Therefore the cpu !=
current_cpu test you add are pointless.

Only once we get to the actual cpufreq driver (intel_pstate and others)
do we run into the fact that we might not be able to service the request
remotely. But since you also add a test there, that is sufficient.