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

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Message-ID: <alpine.DEB.2.11.1509302358120.4500@nanos>
Date:	Thu, 1 Oct 2015 00:02:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chris Metcalf <cmetcalf@...hip.com>
cc:	Andy Lutomirski <luto@...capital.net>,
	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, 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 have yet to understand WHY this loop is there at all. All I've seen
so far is that things might need to settle and that the indicator for
settlement is that the next expiry time of the per cpu timer is set to
KTIME_MAX.

To be blunt, that just stinks. This is duct tape engineering and not
even close to a reasonable design.

Thanks,

	tglx
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