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

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Message-ID: <CAG4TOxNMRR_-bYJV1a_je2HKrpmVuGubH9ks4GdG8Shwa92K0w@mail.gmail.com>
Date:	Wed, 22 Aug 2012 09:10:37 -0700
From:	Roland Dreier <roland@...nel.org>
To:	netdev@...r.kernel.org
Subject: Best way to set kernel thread affinity for handling a socket?

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?

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()).
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.

Thanks!
  Roland
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