linux-kernel - Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks range

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Date:	Tue, 2 Apr 2013 15:09:48 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/4] nohz: New option to force all CPUs in full dynticks range

2013/3/30 Ingo Molnar <mingo@...nel.org>:
>
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
>
>> 2013/3/28 Ingo Molnar <mingo@...nel.org>:
>> >
>> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
>> >
>> >> +config NO_HZ_EXTENDED_ALL
>> >> +       bool "Full dynticks system on all CPUs"
>> >> +       depends on NO_HZ_EXTENDED
>> >> +       help
>> >> +      Force all CPUs to be full dynticks. The range specified in the
>> >> +      nohz_extended boot option will then be ignored.
>> >> +
>> >> +      Note the boot CPU will still be kept outside the range to handle
>> >> +      the timekeeping duty.
>> >
>> > In most cases this will be the first time users doing 'make oldconfig'
>> > will hear about this feature, so I'd not mention noh_extended in such a
>> > primary way at all.
>> >
>> > Instead I'd suggest to explain, in a few simple sentences, what 'full
>> > dynticks' _is_. Then maybe mention this near the end of the next:
>> >
>> >    ( Alternatively this feature can also be enabled via the
>> >      nohz_extended=<cpulist> boot option. If this kernel config option is
>> >      enabled then the nohz_extended boot parameter is ignored. )
>>
>> Ok. Hmm, concerning what full dynticks is, those who run make oldconfig
>> should have passed through CONFIG_NO_HZ_EXTENDED help first so they
>> should know what this is all about, right? I can improve its help text
>> if you think something need more details. Or you think I should add some
>> more reminder in the new config? Also nohz_extended is not mentioned
>> there so I indeed need to fix that. And then do a reminder in the new
>> config. I'll reuse a bit your above sentence.
>
> Kconfig option + boot parameter, or kconfig option + kconfig option, just
> to get a feature to activate, is usability madness.
>
> I think we need _one_ config knob to configure, which, if selected,
> activates all of this feature. Preferably this would be similar to the
> preempt options, like CONFIG_PREEMPT is the most advanced preemption
> model:
>
>           ( ) No Forced Preemption (Server)
>           (X) Voluntary Kernel Preemption (Desktop)
>           ( ) Preemptible Kernel (Low-Latency Desktop)
>
> NO_HZ_EXTENDED or NO_HZ_FULL could be a third variant to:
>
>           ( ) Periodic Timer Ticks
>           (X) Tickless Idle (Dynamic Ticks)
>           ( ) Full Tickless (Extended Dynamic Ticks)
>
> or so - instead of this somewhat confusing hierarchy of config and boot
> options. Good help text would explain the differences between then.
>
> Internally, the 'Tickless Idle' option would cause NO_HZ to be set, while
> 'Full Tickless' would also set NO_HZ, but would also set NO_HZ_EXTENDED.
>
> Furthermore, right now it's hard to enable NO_HZ_EXTENDED, because it
> depends on so many other options. The option is just hidden unless the
> user knows about it and enables 5 other kernel options. Instead the tick
> mode selected should enable (select) any required features - it should
> drive things, not the other way around.
>
> (After all that is done, the boot option is something that augments this
> existing, kconfig driven mechanism.)

All agreed, I'm fixing these issues.
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