linux-kernel - Re: RFC: revert 43fa5460fe60

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Message-ID: <20150224124112.3d4c728c@gandalf.local.home>
Date:	Tue, 24 Feb 2015 12:41:12 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jörn Engel <joern@...estorage.com>
Cc:	Steven Rostedt <srostedt@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Gregory Haskins <ghaskins@...ell.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RFC: revert 43fa5460fe60

On Tue, 24 Feb 2015 09:19:06 -0800
Jörn Engel <joern@...estorage.com> wrote:

> Well, reverting was my first instinct, but for different reasons I think
> it is wrong.  Simply reverting can result in the high priority thread
> moving from one cpu with a running process to a different cpu with a
> running process.  In both cases you may trip over a mole, so nothing
> much is gained.
> 
> But if you know that the destination cpu is idle, you can avoid any
> moles, give or take a small race window maybe.  The moles are still
> present and you still need some debug tool to detect them and fix them
> over time.  But as cpus increase over time, your chances of getting
> lucky in spite of bad kernel code also increase.
> 
> Is that a worthwhile approach, at least for non PREEMPT?

I don't know. Could probably add it if CONFIG_PREEMPT is not set. Just
check if an idle CPU is available in that case and move it, as non
PREEMPT kernels will have long latencies anyway.

-- Steve
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