* Paul E. McKenney [off-list ref] [170718 10:00]:

On Mon, Jul 17, 2017 at 10:41:38PM -0700, Tony Lindgren wrote:

quoted

* Paul E. McKenney [off-list ref] [170717 05:40]:

quoted

On Sun, Jul 16, 2017 at 11:08:07PM -0700, Tony Lindgren wrote:

quoted

* Alex Shi [off-list ref] [170716 16:25]:

quoted

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().

Would you be willing to prototype your usage on v4.12?  It still has
NO_HZ_FULL_SYSIDLE.  You have to enable NO_HZ_FULL in order to enable
NO_HZ_FULL_SYSIDLE at the moment.

Either way, here is the important bit for usage:

	bool rcu_sys_is_idle(void);
	void rcu_sysidle_force_exit(void);

The rcu_sys_is_idle() function returns true if all CPUs other than the
time-keeping CPU (that is, tick_do_timer_cpu, which is usually CPU 0)
are in their idle loop.  Of course, if you invoke rcu_sys_is_idle()
from any CPU other than the time-keeping CPU, you will automatically
get a return value of false.

RCU's idle-exit code already sets state appropriately, but if there
is some other circumstance where you need to force the state machine
out of all-CPUs-idle state, you can call rcu_sysidle_force_exit().

OK sure I'll take a look at some point.

Thanks,

Tony