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

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Message-ID: <x494njzxdd9.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 06 Dec 2012 13:08:18 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm\@kvack.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

Tejun Heo <tj@...nel.org> writes:

> Hmmm... cpu binding usually is done by kthread_bind() or explicit
> set_cpus_allowed_ptr() by the kthread itself.  The node part of the
> API was added later because there was no way to control where the
> stack is allocated and we often ended up with kthreads which are bound
> to a CPU with stack on a remote node.
>
> I don't know.  @node usually controls memory allocation and it could
> be surprising for it to control cpu binding, especially because most
> kthreads which are bound to CPU[s] require explicit affinity
> management as CPUs go up and down.  I don't know.  Maybe I'm just too
> used to the existing interface.

OK, I can understand this line of reasoning.

> 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.

Cheers,
Jeff
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