netdev - Re: [RFC] [PATCH] net: Add support for ndo_select_queue() functions to cache the queue mapping

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Message-Id: <1264702224.2783.18.camel@achroite.uk.solarflarecom.com>
Date:	Thu, 28 Jan 2010 18:10:24 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	linux-net-drivers@...arflare.com, netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] net: Add support for ndo_select_queue()
	functions to cache the queue mapping

On Thu, 2010-01-28 at 23:39 +0530, Krishna Kumar2 wrote:
> > Ben Hutchings <bhutchings@...arflare.com>
> >
> > We think it's worth matching up RX and TX queue selection for a socket
> > when the queues share an interrupt.  Currently the default TX queue
> > selection is unlikely to match RX queue selection.  TX queue selectionc
> > can be overridden by the driver to match RX queue selection, but at the
> > expense of caching.  We found that without caching the cost of
> > recalculating the the hash in software for each packet outweighed the
> > benefit of good queue selection.
> >
> > This adds two simple helper functions: sk_may_set_tx_queue() reports
> > whether the queue mapping should be cacheable for a socket, and
> > netif_sk_tx_queue_set() sets it after validating the queue index.  This
> > will allow drivers to evalulate expensive queue selection functions
> > (such as Toeplitz hash functions) only once for connected sockets, and
> > to avoid doing so for other skbs.
[...]
> I am confused about this - isn't the txq# and rxq# already
> matched due to the check for skb_rx_queue_recorded?

When forwarding, yes.

> sk_dst_cache is also not set for these routing/forwarding
> workloads, this can be relied only for locally connected
> xmits.

Correct, that's exactly what it's for.

> Other than that, I saw netif_sk_tx_queue_set is not called.
> And dev_pick_tx has already capped automatically, you probably
> don't need another here?

Only the return value of ndo_select_queue() is capped; the cached value
is assumed to be valid.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
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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