Re: [RFC PATCH 18/19] cpufreq: remove transition_lock
From: Juri Lelli
Date: Tue Jan 19 2016 - 11:01:37 EST
On 19/01/16 16:30, Peter Zijlstra wrote:
> 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.
Yeah, also cpufreq relies on blocking notifiers (to name one thing). So,
it seems to me quite some things needs to be changed to make it fully
> > 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.
Right, read path is fast, but write path still requires some sort of
locking (malloc, copy and update). So, I'm wondering if this still pays
off for a structure that gets written a lot.
> So on 'sane' hardware with per logical cpu hints you can get away
> without any locks.
But maybe you are saying that there are ways we can make that work :).