netdev - RE: [PATCH] net: fix saving TX flow hash in sock for outgoing connections

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Date:	Wed, 22 Oct 2014 18:35:59 +0000
From:	Sathya Perla <Sathya.Perla@...lex.Com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"therbert@...gle.com" <therbert@...gle.com>
Subject: RE: [PATCH] net: fix saving TX flow hash in sock for outgoing
 connections

> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
> 
> On Wed, 2014-10-22 at 21:42 +0530, Sathya Perla wrote:
> > The commit "net: Save TX flow hash in sock and set in skbuf on xmit"
> > introduced the inet_set_txhash() and ip6_set_txhash() routines to
> calculate
> > and record flow hash(sk_txhash) in the socket structure. sk_txhash is used
> > to set skb->hash which is used to spread flows across multiple TXQs.
> >
> > But, the above routines are invoked before the source port of the
> connection
> > is created. Because of this all outgoing connections that just differ in the
> > source port get hashed into the same TXQ.
> 
> Acked-by: Eric Dumazet <edumazet@...gle.com>
> 
> Are you really using the socket/flow hash to select a TXQ ?
Yes, as I don't have XPS configured on my setup.
netdev_pick_tx() uses the socket/flow hash when XPS is not used.

> 
> Even with this patch, you have a good probability of multiple
> cpus hitting same TXQ.
Agree. Are you suggesting that drivers should automatically
register an XPS configuration? I thought it was upto the user
to enable it...