[PATCH RFC idle 2/3] arm: Avoid invoking RCU when CPU is idle

[Kevin Hilman]

"Paul E. McKenney" <paulmck at linux.vnet.ibm.com> writes:

> On Thu, Feb 02, 2012 at 02:20:26PM -0800, Kevin Hilman wrote:
>> "Paul E. McKenney" <paulmck at linux.vnet.ibm.com> writes:
>> 
>> [...]
>> 
>> >> > The two options I see are:
>> >> > 
>> >> > 1.	Rip tracing out of the inner idle loops and everything that
>> >> > 	they invoke.
>> >> 
>> >> What I suggested above.  But as I said I know sh*t about that tracing 
>> >> implementation so that's an easy suggestion for me to make.
>> >
>> > Works for me as well.  ;-)
>> 
>> While I must admit not having a better suggestion, I for one would vote
>> strongly against removing tracing from the idle path.
>> 
>> Being a PM developer and maintainer, much of the code I work on and
>> maintain happens to be run in the bowels of the idle path.  Not having
>> the ability to trace this code would be a major step backwards IMO.
>
> OK...
>
> What if the tracing code between the rcu_idle_enter() and the
> rcu_idle_exit() had to be enclosed in a wrapper?  For example,
> the tracing in cpuidle_idle_call() might appear as follows:
>
> 	RCU_NONIDLE(
> 		trace_power_start(POWER_CSTATE, next_state, dev->cpu);
> 		trace_cpu_idle(next_state, dev->cpu);
> 	);
>
> 	entered_state = target_state->enter(dev, drv, next_state);
>
> 	RCU_NONIDLE(
> 		trace_power_end(dev->cpu);
> 		trace_cpu_idle(PWR_EVENT_EXIT, dev->cpu);
> 	);
>
> The RCU_NONIDLE() macro would do rcu_idle_exit(), execute its
> argument, then do rcu_idle_enter().  (Credit to Steven Rostedt
> for suggesting this.)  Given the possibility of code invoked both
> from idle and not-idle, I have some changes to rcu to allow nesting
> of rcu_idle_enter() and rcu_idle_exit().
>
> Would that work for you?

Yes, that should work.

And I defintely have examples of code paths that use tracepoints in both
idle and non-idle context (power domains, clocks, etc.) so the changes
to allow nesting will be needed.

Thanks,

Kevin