Re: [PATCH tip/core/rcu 6/9] nohz_full: Add full-system idle states and variables

[Josh Triplett <josh@joshtriplett.org>]

On Sat, Aug 17, 2013 at 06:49:41PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> This commit adds control variables and states for full-system idle.
> The system will progress through the states in numerical order when
> the system is fully idle (other than the timekeeping CPU), and reset
> down to the initial state if any non-timekeeping CPU goes non-idle.
> The current state is kept in full_sysidle_state.
> 
> A RCU_SYSIDLE_SMALL macro is defined, and systems with this number
> of CPUs or fewer move through the states more aggressively.  The idea
> is that the resulting memory contention is less of a problem on small
> systems.  Architectures can adjust this value (which defaults to 8)
> using CONFIG_ARCH_RCU_SYSIDLE_SMALL.
> 
> One flavor of RCU will be in charge of driving the state machine,
> defined by rcu_sysidle_state.  This should be the busiest flavor of RCU.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>

One issue (and one question) below; with the issue addressed,
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index eab81da..64a05b9f 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu)
>  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
>  
>  /*
> + * Handle small systems specially, accelerating their transition into
> + * full idle state.  Allow arches to override this code's idea of
> + * what constitutes a "small" system.
> + */
> +#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL

I don't see any Kconfig creating this new config option.

Also, why not simply define this config option unconditionally, with a
default of 8, and then use its value directly?

> +static int __maybe_unused full_sysidle_state; /* Current system-idle state. */
> +#define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
> +#define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
> +#define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
> +#define RCU_SYSIDLE_FULL	3	/* All CPUs idle, ready for sysidle. */
> +#define RCU_SYSIDLE_FULL_NOTED	4	/* Actually entered sysidle state. */

Perhaps there's a kernel style rule I'm not thinking of that makes it
verboten, but: why not use an enum for a state variable like this?

- Josh Triplett