netdev - Re: [PATCH 1/2] net: Toeplitz library functions

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Message-ID: <20130924194215.GA4446@order.stressinduktion.org>
Date:	Tue, 24 Sep 2013 21:42:15 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>
Subject: Re: [PATCH 1/2] net: Toeplitz library functions

On Tue, Sep 24, 2013 at 11:02:11AM -0700, Tom Herbert wrote:
> On Tue, Sep 24, 2013 at 10:34 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Tue, 2013-09-24 at 10:03 -0700, Tom Herbert wrote:
> >> On Tue, Sep 24, 2013 at 9:46 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> >> > On Tue, 2013-09-24 at 09:35 -0700, Tom Herbert wrote:
> >> >
> >> >> We should really be using rxhash for that anyway, eliminate this
> >> >> ehashfn.  This would entail adding rxhash argument in the various
> >> >> udp_lookup functions.
> >> >
> >> > Nope : Some NICs provide UDP rxhash only using L3  (source IP,
> >> > destination IP), not L4 (adding source & destination ports)
> >> >
> >> Then the NIC won't set l4_rxhash and we'll rehash over 4-tuple when
> >> skb_get_rxhash is called.
> >
> > Yes, but then in this case you add cpu cycles for no reason.
> >
> > If you have multiqueue NIC, you do not use RPS/RFS, so skb->rxhash might
> > be 0
> >
> > hash = inet_ehashfn(net, daddr, hnum, saddr, sport);
> >
> > is faster than the whole flow dissection game.
> >
> But if we already have valid l4_rxhash we're just wasting time
> recomputing it (very like the same value).  So just do:
> 
> if (skb->l4_hash)
>     hash = skb->rxhash
> else
>     hash = inet_ehashfn(net, daddr, hnum, saddr, sport);
> 
> Even if we go the inet_ehashfn route in the packet, it's makes sense
> to store this in skb->rxhash so that subsequent functions don't need
> to compute the hash
> 
> By the way, I believe there is an insidious in using the same hash
> value or even related hash values in multiple places for steering.
> Since we typically do something like hash % numqueues, we introduce
> bias if multiple steering levels look at the same bits.  With
> SO_REUSEPORT this might actually be beneficial since we could get port
> selection to match RSS locality, but this is not by design!

Wouldn't this also need infrastructure to sync the keys over multiple network
cards to have best benefits?

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