linux-kernel - Re: [patch,v2] bdi: add a user-tunable cpu

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Date:	Thu, 6 Dec 2012 10:13:10 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Zach Brown <zab@...hat.com>,
	Peter Zijlstra <pzijlstr@...hat.com>, Ingo <mingo@...hat.com>
Subject: Re: [patch,v2] bdi: add a user-tunable cpu_list for the bdi flusher
 threads

Hello,

On Thu, Dec 06, 2012 at 01:08:18PM -0500, Jeff Moyer wrote:
> > As for the original patch, I think it's a bit too much to expose to
> > userland.  It's probably a good idea to bind the flusher to the local
> > node but do we really need to expose an interface to let userland
> > control the affinity directly?  Do we actually have a use case at
> > hand?
> 
> Yeah, folks pinning realtime processes to a particular cpu don't want
> the flusher threads interfering with their latency.  I don't have any
> performance numbers on hand to convince you of the benefit, though.

What I don't get is, RT tasks win over bdi flushers every time and I'm
skeptical allowing bdi or not on a particular CPU would make a big
difference on non-RT kernels anyway.  If the use case calls for
stricter isolation, there's isolcpus.  While I can see why someone
might think that they need something like this, I'm not sure it's
actually something necessary.

And, even if it's actually something necessary, I think we'll probably
be better off with adding a mechanism to notify userland of new
kthreads and let userland adjust affinity using the usual mechanism
rather than adding dedicated knobs for each kthread users.

Thanks.

-- 
tejun
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