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

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Date:	Mon, 15 Apr 2013 16:11:43 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Ingo Molnar <mingo@...nel.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.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 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

On Mon, 15 Apr 2013, Ingo Molnar wrote:

> > Ok. But all these complicated things would go away if we had an option
> > CONFIG_LOWLATENCY and then everything would just follow the best setup
> > possible given the hardware. Would remove a lot of guesswork and a lot of
> > knobs.
>
> In that sense CONFIG_NO_HZ_FULL is such a flag as well, which, like
> CONFIG_PREEMPT_RT, tries to preconfigure the kernel correctly.

It is too specific. Switching off HZ is one in a set of measures that one
woiuld take. F.e. the page allocator will want to do reclaim on the
sacrificial processor, the vmstatistics would run their monitoring thread
there etc etc.

> But we have to be careful not to use a too highlevel flag for that. If the user
> meant 'low latency' to mean 'low latency IRQ execution' - then enabling
> CONFIG_NO_HZ_FULL might achieve the opposite, it adds overhead to the IRQ paths.

Hmm.... Looks too specific for my taste. What the user wants is less OS
noise (or maybe some other terms that describes that the user wants
minimal disturbances for his code).

	CONFIG_OS_NOISE_REDUCTION

?

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