Re: Regression with suspicious RCU usage splats with cpu_pm change

[Tony Lindgren <tony@atomide.com>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> [170717 05:40]:
> On Sun, Jul 16, 2017 at 11:08:07PM -0700, Tony Lindgren wrote:
> > * Alex Shi <alex.shi@linaro.org> [170716 16:25]:
> > > I reused the rcu_irq_enter_irqson() from RCU_NONIDLE to avoid this issue. 
> > > It works fine.
> > > 
> > > Tony, Could you like to give a tested-by if this patch works for you.
> > 
> > Yeah that keeps things booting for me with no splats so:
> > 
> > Tested-by: Tony Lindgren <tony@atomide.com>
> > 
> > In general, it seems we're missing the knowledge in Linux kernel
> > of when the entire system is idle. Right now it seems that only
> > cpuidle_coupled knows that?
> > 
> > We could probably simplify things by adding some PM state for
> > entire system idle. Then cpuidle code and timer code could use
> > that to test when it's safe to do whatever the SoC needs to do
> > to enter deeper power states.
> > 
> > If we already have something like that, please do let me know :)
> 
> Well, we used to have CONFIG_NO_HZ_FULL_SYSIDLE, which detected
> full-system idle lazily so as to avoid scalability bottlenecks.
> https://lwn.net/Articles/558284/
> 
> No one was using it, so I removed it last merge window.  The
> patch that removed it is at sysidle.2017.05.11a, which can
> probably still be reverted cleanly.  Or just use v4.11 or earlier.

OK thanks for the pointer, for reference that commit is
fe5ac724d81a ("rcu: Remove nohz_full full-system-idle state
machine").

For a potential user, I think we could use it for example in
cpuidle_enter_state_coupled() + omap_enter_idle_coupled() where
we try to figure out if the system is fully idle before calling
tick_broadcast_enter().

Regards,

Tony