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

[Steven Rostedt]

On Thu, 2012-02-02 at 16:49 -0600, Rob Herring wrote:
> On 02/02/2012 04:20 PM, 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.
> 
> How is it a step backwards if it is already broken. Obviously you
> haven't actually used any tracing here because it doesn't work right
> with things as is.
> 
> What exactly do you want to trace at this level. By the point you are in
> this code, the path is somewhat known and problems you have are likely
> h/w issues. If you are trying to go thru a very precise sequence of
> saving cpu state and flushing caches, you don't want calls out to
> tracing code that could very easily change the behavior.
> 
> It would be nice to understand a real example of how to hit this
> problem. Why type of tracing? Are there other known examples?

Well, these are the tracepoints in question:

trace_power_start(), trace_cpu_idle(), and trace_power_end()

But this is only used by powertop, and I'm sure nobody would mind if you
rip these tracepoints out, causing powertop to no longer work. (that's
why we have 4 bytes per tracepoint wasting space in every trace event).

Anyway, one answer is (and I was talking with Paul about this on IRC) is
that we can create a special "TRACE_EVENT_IDLE()" that will explicitly
call "rcu_idle_exit/enter()" as it expects to be called with it off.

This should solve most issues I believe.

-- Steve