linux-kernel - Re: [PATCH v7 07/11] arch/x86: enable task isolation functionality

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Date:	Wed, 30 Sep 2015 15:11:44 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.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>,
	"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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v7 07/11] arch/x86: enable task isolation functionality

On Wed, Sep 30, 2015 at 3:02 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 30 Sep 2015, Chris Metcalf wrote:
>> So for now, if a task-isolation thread sets up a timer,
>> they're screwed: so, don't do that.  And it's really not part of
>> the typical programming model for these kinds of userspace
>> drivers anyway, so it's pretty reasonable to forbid it.
>
> There is a difference between forbidding it and looping for 10 minutes
> in the kernel.

I don't even like forbidding it.  Setting timers seems like an
entirely reasonable thing for even highly RT or isolated programs to
do, although admittedly they can do it on a non-RT thread and then
kick the RT thread when they're ready.

Heck, even without the TSC deadline timer, the kernel could, in
principle, support that use case by having whatever core is doing
housekeeping keep kicking the can forward until it's time to IPI the
isolated core because it needs to wake up.

--Andy
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