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

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Message-ID: <CAFTL4hzQfodVP4O+nir1XwQ2QkUUqrfr_dBpWL0635f0u4oRBA@mail.gmail.com>
Date:	Thu, 28 Mar 2013 14:43:41 +0100
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

2013/3/28 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.

>
> Btw., while at it - I think the nohz_extended boot parameter should _not_
> be ignored. The .config option should simply provide a default, for the
> case that the user does not have any boot option specified.

Agreed.

>
> If the user specifies a boot option - say to work around a rare installer
> hang in a distro install kernel - then we very much _dont_ want to ignore
> it.
>
> So please put some effort into how this feature is going to interface with
> and be represented to actual users and distro kernel maintainers.

Ok, will improve that.

Thanks.
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