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Date: Fri, 8 Nov 2013 20:12:46 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Mike Galbraith <bitbucket@online.de>,
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        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Mike Frysinger <vapier@gentoo.org>
Subject: Re: [PATCH] kmod: Run usermodehelpers only on cpus allowed for
 kthreadd V2
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On Fri, Nov 08, 2013 at 05:05:35PM +0000, Christoph Lameter wrote:
> On Fri, 8 Nov 2013, Frederic Weisbecker wrote:
> 
> > But it looks like it always end up calling a workqueue. May be I missed something though.
> >
> > Now we can argue that this workqueue seem to create kernel threads, which in turn create other kernel thread (uhh?)
> > and I don't know if those inherit the kworker affinity. But from a quick look, it seems to me that
> > this is what we want.
> 
> Right. The problem is that the affinity cannot be inherited since
> usermodehelper may be called from a workqueue or other restricted kernel
> context. The main point of the usermodehelper logic is to work itself out
> of the restrictions of the context in which is was called to be able to
> fork off a kernel thread that then can call a userspace helper program.

I understand, but why not solving that from the workqueue affinity? We want to
solve the issue of unbound workqueues in CPU isolation anyway.

> 
> I want to restrict on which processors this working out of the limiting
> context can occur. It should not occur on low latency processors nor
> should user space stuff be run there.
> 
> Without this patch we see various processes being sprinkled over all the
> processors in the system when usermodehelper is invoked.

I got that, and I too am interested in solving this problem. I'm not fighting
against the purpose of this patch but against the way we solve that.