Re: [RFC][PATCH] cpu_pm: Remove RCU abuse

[Ulf Hansson]

On Thu, 3 Sep 2020 at 17:08, <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Sep 03, 2020 at 04:36:35PM +0200, Ulf Hansson wrote:
> > On Thu, 3 Sep 2020 at 15:53, <peterz@xxxxxxxxxxxxx> wrote:
> > >  static int cpu_pm_notify(enum cpu_pm_event event)
> > >  {
> > >         int ret;
> > >
> > > +       lockdep_assert_irqs_disabled();
> >
> > Nitpick, maybe the lockdep should be moved to a separate patch.
>
> Well, the unregister relies on IRQs being disabled here, so I figured
> asserting this was a good thing ;-)

Okay, make sense then.

>
> Starting the audit below, this might not in fact be true, which then
> invalidates the unregister implementation. In particular the notifier in
> arch/arm/kernel/hw_breakpoint.c seems to unconditionally enable IRQs.

I see.

>
> > > +       ret = raw_notifier_call_chain(&cpu_pm_notifier_chain, event, NULL);
> >
> > Converting to raw_notifiers seems reasonable - if we need to avoid the
> > RCU usage.
> >
> > My point is, I wonder about if the notifier callbacks themselves are
> > safe from RCU usage. For example, I would not be surprised if tracing
> > is happening behind them.
>
> A bunch of them seem to call into the clk domain stuff, and I think
> there's tracepoints in that.
>
> > Moreover, I am not sure that we really need to prevent and limit
> > tracing from happening. Instead we could push rcu_idle_enter|exit()
> > further down to the arch specific code in the cpuidle drivers, as you
> > kind of all proposed earlier.
>
> Well, at some point the CPU is in a really dodgy state, ISTR there being
> ARM platforms where you have to manually leave the cache coherency
> fabric and all sorts of insanity. There should be a definite cut-off on
> tracing before that.

That's probably the case for some platforms, but I don't see why we
need to make everybody "suffer".

>
> Also, what is the point of all this clock and power domain callbacks, if
> not to put the CPU into an extremely low power state, surely you want to
> limit the amount of code that's ran when the CPU is in such a state.
>
> > In this way, we can step by step, move to a new "version" of
> > cpu_pm_enter() that doesn't have to deal with rcu_irq_enter_irqson(),
> > because RCU hasn't been pushed to idle yet.
>
> That should be easy enough to audit. The thing is that mainline is now
> generating (debug) splats, and some people are upset with this.
>
> If you're ok with ARM not being lockdep clean while this is being
> reworked I'm perfectly fine with that.

I think the splats can easily be fixed. Short term.

Adding RCU_NONIDLE (or similar) around pm_runtime calls in
psci_enter_domain_idle_state() does the trick. I have a patch for
that, it's tested and ready. Let me send it out.

Perhaps we should just accept that this is needed, as to allow us to
move step by step into a better situation, while also avoiding the
current splats.

>
> (There used to be a separate CONFIG for RCU-lockdep, but that seems to
> have been removed)

I don't think that would help. Infrastructure for testing will just
enable that Kconfig anyway still complains to us.

Kind regards
Uffe