netdev - Re: Best way to set kernel thread affinity for handling a socket?

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Message-ID: <1345652493.2709.36.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Wed, 22 Aug 2012 17:21:33 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Roland Dreier <roland@...nel.org>
CC:	<netdev@...r.kernel.org>
Subject: Re: Best way to set kernel thread affinity for handling a socket?

On Wed, 2012-08-22 at 09:10 -0700, Roland Dreier wrote:
> Hi everyone,
> 
> Let's say I have kernel code that's sitting in a loop doing
> kernel_accept() on a TCP socket.  As each connection comes in, it
> forks off a kernel thread to deal with that socket.
> 
> If I have a modern NIC with RSS and multiple queues, each TCP flow is
> going to be steered to one queue, which is probably bound to one CPU.
> So when I fork off that kernel thread, I'd like to bind it to the CPU
> where its NIC queues are going to be processed.  My question is, how
> do I find out which CPU that is?  Is there anything in the new socket
> structure I get back from kernel_accept() that I can look at to know
> which CPU the packets came in on?

With RFS we try to do the reverse: move the packets to match the socket
user.   But it's not (yet) turned on by default.  See
Documentation/networking/scaling.txt

> I'm thinking about this in the context of the kernel's iSCSI target
> code (drivers/target/iscsi), which creates threads to handle each
> iSCSI connection and sets their CPU affinity pretty much randomly
> (well, based on some "thread id", cf iscsit_thread_get_cpumask()).

Why set the affinity at all?

> And with a modern NIC, this leads to packets being received on one CPU
> but the data being consumed on another CPU, all the time, which is
> obviously far from optimal.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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