Message-ID: <20130818031341.GK28923@leaf>
Date:	Sat, 17 Aug 2013 20:13:41 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 0/9] sysidle changes for v3.12

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@...htriplett.org>
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