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

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Message-ID: <20131017160726.GJ28963@localhost.localdomain>
Date:	Thu, 17 Oct 2013 18:07:28 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	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>,
	Andrew Morton <akpm@...ux-foundation.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 Thu, Oct 17, 2013 at 03:24:36PM +0000, Christoph Lameter wrote:
> On Thu, 17 Oct 2013, Frederic Weisbecker wrote:
> 
> > > -	/* We can run anywhere, unlike our parent keventd(). */
> > > -	set_cpus_allowed_ptr(current, cpu_all_mask);
> > > +	/* We can run anywhere kthreadd can run */
> > > +	set_kthreadd_affinity();
> >
> > If you really want your isolated CPUs to be undisturbed, you need this affinity to
> > be set before the creation of this usermode helper. Otherwise it's going to run to any random
> > place before you call the above function.
> >
> > Is there now way to control the kworker affinity which create this usermode helpers threads
> > instead?
> 
> The invocation is coming from keventd so the affinity seems to be set to
> a different cpu before we get the thread moved.

Looking at alloc_singlethread_workqueue(), it seems to create a kthread by using
kthread_create(). So I guess the resulting works have the affinity of that kthread.
And thus call_usermodehelper() initially has that affinity, until it spreads it explicitly
to all CPUs.

Couldn't we instead make kthread children (those created with kthread_create()) to inherit
kthread initial affinity? Currently kthread's children have cpu_all_mask. We could change
that behaviour. This way the initial kthread affinity could be inherited all along.

> 
> A tsc loop is running on the processor that gets hit by the usermodehelper spawns.
> 
> We see spawning by the tty notifier (when typing on a vt) as well as by a
> mellanox driver (periodically).> 
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