linux-kernel - Re: [PATCH] tilegx: fix some issues in the SW TSO support

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Date:	Thu, 25 Oct 2012 20:36:46 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Chris Metcalf <cmetcalf@...era.com>
CC:	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tilegx: fix some issues in the SW TSO support

On Thu, 2012-10-25 at 14:16 -0400, Chris Metcalf wrote:
> On 10/25/2012 1:51 PM, Ben Hutchings wrote:
> > On Thu, 2012-10-25 at 13:25 -0400, Chris Metcalf wrote:
> >> This change correctly computes the header length and data length in
> >> the fragments to avoid a bug where we would end up with extremely
> >> slow performance.  Also adopt use of skb_frag_size() accessor.
> > [...]
> >
> > By the way, since you're doing soft-TSO you should probably set
> > net_device::gso_max_segs, as explained in:
> >
> > commit 30b678d844af3305cda5953467005cebb5d7b687
> > Author: Ben Hutchings <bhutchings@...arflare.com>
> > Date:   Mon Jul 30 15:57:00 2012 +0000
> >
> >     net: Allow driver to limit number of GSO segments per skb
> 
> We currently have a hard limit of 2048 equeue entries (effectively,
> segments) per interface.  The commit message suggests 861 is the largest
> number we're likely to see, so I think we're OK from a correctness point of
> view.
>
> But, perhaps, we could end up with multiple cores trying to push
> separate flows each with this tiny MSS issue, and they would then be
> contending for the 2048 equeue entries, and performance might suffer.  I
> don't have a good instinct on what value we should choose to set here; I
> see that sfc uses 100.
>
> So, we could do nothing since it seems we're technically safe; we could say
> 2048 to be explicit; we could pick a random fraction of the full size to
> help avoid contention effects, like 1024 or 512; or we could mimic sfc and
> just say 100.  What do you think?

You need at least 2 descriptors per segment, plus 1 for each fragment
boundary, so the maximum would be something like
(2048 - MAX_SKB_FRAGS) / 2 ~= 1015.  So the worst-case skb can fit in
the queue, but it requires so much of the queue space that the queue
might not become sufficiently empty before the watchdog timer fires.

There's a simple way to test this - configure the 'attacker' host and
interface like this:

# ip link set $IFACE mtu 128
# sysctl net.ipv4.route.min_adv_mss=88

and then use something like netperf to generate a high bandwidth stream
of TCP traffic toward it from the tilegx interface.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
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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