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Date:	Sat, 5 Mar 2016 13:31:58 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Chris Metcalf <cmetcalf@...lanox.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 05/12] task_isolation: support
 CONFIG_TASK_ISOLATION_ALL


* Chris Metcalf <cmetcalf@...lanox.com> wrote:

> On 03/03/2016 01:34 PM, Andi Kleen wrote:
> >Chris Metcalf <cmetcalf@...hip.com> writes:
> >>+config TASK_ISOLATION_ALL
> >>+	bool "Provide task isolation on all CPUs by default (except CPU 0)"
> >>+	depends on TASK_ISOLATION
> >>+	help
> >>+	 If the user doesn't pass the task_isolation boot option to
> >>+	 define the range of task isolation CPUs, consider that all
> >>+	 CPUs in the system are task isolation by default.
> >>+	 Note the boot CPU will still be kept outside the range to
> >>+	 handle timekeeping duty, etc.
> >That seems like a very dangerous Kconfig option.
> >"CONFIG_BREAK_EVERYTHING"
> >If someone sets that by default they will have a lot of trouble.
> >
> >I wouldn't add that, make it a run time option only.
> 
> So you were thinking, allow a special boot syntax "task_isolation=all",
> which puts all the cores into task isolation mode except the boot core?
> 
> My original argument was that it was so parallel to the existing
> CONFIG_NO_HZ_FULL_ALL option that it just made sense to do it,
> and some testers complained about having to specify the precise
> cpu range, so this seemed like an easy fix.

Yes, it's absolutely legitimate to offer boot options as Kconfig options as well - 
in fact that will get things like randconfig bootups stumble upon them and do some 
free testing for you. Just ignore Andi's nonsensical objection.

One day we'll have a unified boot parameter/Kconfig/sysctl mechanism, so that it 
will be possible to say things like this on the boot command line:

  CONFIG_NO_HZ_FULL_ALL=y

... which will eliminate quite a bit of the current schizm between Kconfig and 
boot time parameters.

Thanks,

	Ingo

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