Re: [RFC PATCH 18/19] cpufreq: remove transition_lock

From: Peter Zijlstra
Date: Tue Jan 19 2016 - 10:30:21 EST

On Tue, Jan 19, 2016 at 02:42:33PM +0000, Juri Lelli wrote:
> On 19/01/16 15:00, Peter Zijlstra wrote:
> > On Wed, Jan 13, 2016 at 10:21:31AM -0800, Michael Turquette wrote:
> > > RCU is absolutely not a magic bullet or elixir that lets us kick off
> > > DVFS transitions from the schedule() context. The frequency transitions
> > > are write-side operations, as we invariably touch struct cpufreq_policy.
> > > This means that the read-side stuff can live in the schedule() context,
> > > but write-side needs to be kicked out to a thread.
> >
> > Why? If the state is per-cpu and acquired by RCU, updates should be no
> > problem at all.
> >
> > If you need inter-cpu state, then things get to be a little tricky
> > though, but you can actually nest a raw_spinlock_t in there if you
> > absolutely have to.
> >
> We have at least two problems. First one is that state is per frequency
> domain (struct cpufreq_policy) and this usually spans more than one cpu.
> Second one is that we might need to sleep while servicing the frequency
> transition, both because platform needs to sleep and because some paths
> of cpufreq core use sleeping locks (yes, that might be changed as well I
> guess). A solution based on spinlocks only might not be usable on
> platforms that needs to sleep, also.

Sure, if you need to actually sleep to poke the hardware you've lost and
you do indeed need the kthread thingy.

> Another thing that I was thinking of actually is that since struct
> cpufreq_policy is updated a lot (more or less at every frequency
> transition), is it actually suitable for RCU?

That entirely depends on how 'hard' it is to 'replace/change' the
cpufreq policy.

Typically I envision that to be very hard and require mutexes and the
like, in which case RCU can provide a cheap lookup and existence.

So on 'sane' hardware with per logical cpu hints you can get away
without any locks.