linux-kernel - Re: [PATCH RFC nohz_full 1/7] nohz_full: Add Kconfig parameter for scalable detection of all-idle state

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Date:	Mon, 5 Aug 2013 03:04:55 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
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@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, sbw@....edu
Subject: Re: [PATCH RFC nohz_full 1/7] nohz_full: Add Kconfig parameter for
 scalable detection of all-idle state

On Fri, Jul 26, 2013 at 04:19:18PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> At least one CPU must keep the scheduling-clock tick running for
> timekeeping purposes whenever there is a non-idle CPU.  However, with
> the new nohz_full adaptive-idle machinery, it is difficult to distinguish
> between all CPUs really being idle as opposed to all non-idle CPUs being
> in adaptive-ticks mode.  This commit therefore adds a Kconfig parameter
> as a first step towards enabling a scalable detection of full-system
> idle state.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/time/Kconfig | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index 70f27e8..a613c2a 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -134,6 +134,29 @@ config NO_HZ_FULL_ALL
>  	 Note the boot CPU will still be kept outside the range to
>  	 handle the timekeeping duty.
>  
> +config NO_HZ_FULL_SYSIDLE
> +	bool "Detect full-system idle state for full dynticks system"
> +	depends on NO_HZ_FULL
> +	default n
> +	help
> +	 At least one CPU must keep the scheduling-clock tick running
> +	 for timekeeping purposes whenever there is a non-idle CPU,
> +	 where "non-idle" includes CPUs with a single runnable task
> +	 in adaptive-idle mode.

"adaptive-idle" is particularly confusing here. How about this:

    'where "non-idle" also includes dynticks CPUs as long they are
    running non-idle tasks.' 

          Because the underlying adaptive-tick
> +	 support cannot distinguish between all CPUs being idle and
> +	 all CPUs each running a single task in adaptive-idle mode,

s/adaptive-idle/dynticks

Thanks.

> +	 the underlying support simply ensures that there is always
> +	 a CPU handling the scheduling-clock tick, whether or not all
> +	 CPUs are idle.  This Kconfig option enables scalable detection
> +	 of the all-CPUs-idle state, thus allowing the scheduling-clock
> +	 tick to be disabled when all CPUs are idle.  Note that scalable
> +	 detection of the all-CPUs-idle state means that larger systems
> +	 will be slower to declare the all-CPUs-idle state.
> +
> +	 Say Y if you would like to help debug all-CPUs-idle detection.
> +
> +	 Say N if you are unsure.
> +
>  config NO_HZ
>  	bool "Old Idle dynticks config"
>  	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
> -- 
> 1.8.1.5
> 
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