Date: Sat, 5 Mar 2016 13:31:58 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Andi Kleen <andi@firstfloor.org>, Gilad Ben Yossef <giladb@ezchip.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Tejun Heo <tj@kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10 05/12] task_isolation: support
 CONFIG_TASK_ISOLATION_ALL
Message-ID: <20160305123158.GA25399@gmail.com>
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 <87mvqfe7h3.fsf@tassilo.jf.intel.com>
 <56D8931E.4060205@mellanox.com>
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* Chris Metcalf <cmetcalf@mellanox.com> wrote:

> On 03/03/2016 01:34 PM, Andi Kleen wrote:
> >Chris Metcalf <cmetcalf@ezchip.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