Re: [PATCH tip/core/rcu 0/9] sysidle changes for v3.12

[Josh Triplett <josh@joshtriplett.org>]

On Sat, Aug 17, 2013 at 06:49:18PM -0700, Paul E. McKenney wrote:
> Hello!
> 
> Whenever there is at least one non-idle CPU, it is necessary to
> periodically update timekeeping information.  Before NO_HZ_FULL, this
> updating was carried out by the scheduling-clock tick, which ran on
> every non-idle CPU.  With the advent of NO_HZ_FULL, it is possible
> to have non-idle CPUs that are not receiving scheduling-clock ticks.
> This possibility is handled by assigning a timekeeping CPU that continues
> taking scheduling-clock ticks.
> 
> Unfortunately, timekeeping CPU continues taking scheduling-clock
> interrupts even when all other CPUs are completely idle, which is
> not so good for energy efficiency and battery lifetime.  Clearly, it
> would be good to turn off the timekeeping CPU's scheduling-clock tick
> when all CPUs are completely idle.  This is conceptually simple, but
> we also need good performance and scalability on large systems, which
> rules out implementations based on frequently updated global counts of
> non-idle CPUs as well as implementations that frequently scan all CPUs.
> Nevertheless, we need a single global indicator in order to keep the
> overhead of checking acceptably low.
> 
> The chosen approach is to enforce hysteresis on the non-idle to
> full-system-idle transition, with the amount of hysteresis increasing
> linearly with the number of CPUs, thus keeping contention acceptably low.
> This approach piggybacks on RCU's existing force-quiescent-state scanning
> of idle CPUs, which has the advantage of avoiding the scan entirely on
> busy systems that have high levels of multiprogramming.  This scan
> takes per-CPU idleness information and feeds it into a state machine
> that applies the level of hysteresis required to arrive at a single
> full-system-idle indicator.
> 
> The individual patches are as follows:
> 
> 1.	Eliminate unused APIs that were intended for adaptive ticks.
> 
> 2.	Add documentation covering the testing of nohz_full.
> 
> 3.	Add a CONFIG_NO_HZ_FULL_SYSIDLE Kconfig parameter to enable
> 	this feature.  Kernels built with CONFIG_NO_HZ_FULL_SYSIDLE=n
> 	act exactly as they do today.
> 
> 4.	Add new fields to the rcu_dynticks structure that track CPU-idle
> 	information.  These fields consider CPUs running usermode to be
> 	non-idle, in contrast with the existing fields in that structure.
> 
> 5.	Track per-CPU idle states.
> 
> 6.	Add full-system idle states and state variables.
> 
> 7.	Expand force_qs_rnp(), dyntick_save_progress_counter(), and
> 	rcu_implicit_dynticks_qs() APIs to enable passing full-system
> 	idle state information.
> 
> 8.	Add full-system-idle state machine.
> 
> 9.	Force RCU's grace-period kthreads onto the timekeeping CPU.

Comments on 4, 5, and 6; for 1-3 and 7-9,
Reviewed-by: Josh Triplett <josh@joshtriplett.org>