linux-kernel - Re: [PATCH] kmod: Run usermodehelpers only on cpus allowed for kthreadd

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Date:	Sun, 20 Oct 2013 18:00:24 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <bitbucket@...ine.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Frysinger <vapier@...too.org>
Subject: Re: [PATCH] kmod: Run usermodehelpers only on cpus allowed for
 kthreadd

On Fri, 18 Oct 2013, Frederic Weisbecker wrote:

> Makes sense yeah. In fact what I'm mostly concerned about is that we should
> set the affinity of __call_usermodehelper threads through inheritance from
> a parent rather than making it setting its affinity itself. Because in the latter case,
> the usermodehelper thread can run anywhere until it sets its affinity. Whether
> this little window of global affinity is short or not, this defeats the initial purpose
> of this patch that is about isolating CPUs and having them undisturbed.
>
> May be we can do that by setting the affinity of the "khelper" workqueue?

The "parent" for usemodehelper in this case is keventd. The worker queue
item is triggered on a particular processor and thats fine because that is
the result of an OS action or a device irq action. These can already be
avoided. What is not ok is that a process makes a move onto a hardware
thread where we want to have the least OS holdoffs possible.

Setting it via khelper would be fine. Any objections?

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