netdev - Re: [PATCH v5 net-next 0/2] patchset: Support for configurable RSS hash key

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Message-ID: <1395412393.2770.48.camel@deadeye.wl.decadent.org.uk>
Date:	Fri, 21 Mar 2014 14:33:13 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	David Miller <davem@...emloft.net>
Cc:	VenkatKumar.Duvvuru@...lex.com, netdev@...r.kernel.org
Subject: Re: [PATCH v5 net-next 0/2] patchset: Support for configurable RSS
 hash key

On Wed, 2014-03-19 at 15:59 -0400, David Miller wrote:
> From: Venkat Duvvuru <VenkatKumar.Duvvuru@...lex.com>
> Date: Mon, 17 Mar 2014 18:01:33 +0530
> 
> > NIC drivers that support RSS use either a hard-coded value or a random value for
> > the RSS hash key. Irrespective of the type of the key used, the user would want
> > to change the hash key if he/she is not satisfied with the effectiveness of the
> > default hash-key in spreading the incoming flows evenly across the RSS queues.
> > 
> > This patch set adds support for configuring the RSS hash-key via the ethtool
> > interface using -X option.
> 
> I apologize, but I really dislike this.  For several reasons.
> 
> First, why aren't we adding _just_ a RSS hash changing interface?
>
> We already have an interface for changing the indirection table,
> there is absolutely not need to add a second interface that supports
> both indirection table _plus_ hash modifications.

That's what I asked for, because I see the hash key and indirection
table as being a single logical object (RSS context).

> And combining these two is what leads to this hard to audit, ugly,
> data structure layout.
> 
> There's the indirection table at some offset, then the key at some
> other offset.  This makes it impossible to impose type checking
> of any kind on both objects.
[...]

Which is why I previously suggested making the ethtool core find the two
arrays and pass those into the driver operations.  We do that for other
ethtool operations that use even a single variable-length array.

Ben.

-- 
Ben Hutchings
One of the nice things about standards is that there are so many of them.

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