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Re: [PATCH 1/2] staging: octeon-ethernet: don't assume that CPU 0 is spe

To: Aaro Koskinen <aaro.koskinen@iki.fi>
Subject: Re: [PATCH 1/2] staging: octeon-ethernet: don't assume that CPU 0 is special
From: David Daney <ddaney@caviumnetworks.com>
Date: Mon, 30 Sep 2013 14:08:31 -0700
Cc: <devel@driverdev.osuosl.org>, <linux-mips@linux-mips.org>, David Daney <david.daney@cavium.com>, <richard@nod.at>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On 09/30/2013 12:56 PM, Aaro Koskinen wrote:
Hi,

On Mon, Sep 30, 2013 at 12:41:59PM -0700, David Daney wrote:
On 09/30/2013 12:35 PM, Aaro Koskinen wrote:
No, the original logic was already broken. The code assumed that the
NAPI scheduled by the driver init gets executed always on CPU 0. The
IRQ got enabled just because we are lucky.

No.  The default affinity for all irqs is CPU0 for just this reason.
So there was no luck involved.

According the Kconfig, this driver can be compiled as a module:

config OCTEON_ETHERNET
        tristate "Cavium Networks Octeon Ethernet support"
[...]
        To compile this driver as a module, choose M here.  The module
        will be called octeon-ethernet.

What guarantees that CPU0 is around (or the smp_affinity is at its
default value) by the time user executes modprobe?

Nothing enforced by the kernel. Just don't take CPU0 off-line and you should be good to go.

There is a lot of room for improvement in the driver.

Really this whole thing of starting NAPI on multiple CPUs for the same input queue is not good. It leads to loss of packet ordering when forwarding.

What we really want is to have one POW group (input queue) per CPU and only run one CPU per group. If we did that, we wouldn't have all these affinity issues.

David Daney

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