Re: [Xen-devel] igb and bnx2: "NETDEV WATCHDOG: transmit queue timed out" when skb has huge linear buffer

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 31/01/14 18:56, Wei Liu wrote:
> On Thu, Jan 30, 2014 at 07:08:11PM +0000, Zoltan Kiss wrote:
> > Hi,
> >
> > I've experienced some queue timeout problems mentioned in the
> > subject with igb and bnx2 cards. I haven't seen them on other cards
> > so far. I'm using XenServer with 3.10 Dom0 kernel (however igb were
> > already updated to latest version), and there are Windows guests
> > sending data through these cards. I noticed these problems in XenRT
> > test runs, and I know that they usually mean some lost interrupt
> > problem or other hardware error, but in my case they started to
> > appear more often, and they are likely connected to my netback grant
> > mapping patches. These patches causing skb's with huge (~64kb)
> > linear buffers to appear more often.
> > The reason for that is an old problem in the ring protocol:
> > originally the maximum amount of slots were linked to MAX_SKB_FRAGS,
> > as every slot ended up as a frag of the skb. When this value were
> > changed, netback had to cope with the situation by coalescing the
> > packets into fewer frags.
> > My patch series take a different approach: the leftover slots
> > (pages) were assigned to a new skb's frags, and that skb were
> > stashed to the frag_list of the first one. Then, before sending it
> > off to the stack it calls skb = skb_copy_expand(skb, 0, 0,
> > GFP_ATOMIC, __GFP_NOWARN), which basically creates a new skb and
> > copied all the data into it. As far as I understood, it put
> > everything into the linear buffer, which can amount to 64KB at most.
> > The original skb are freed then, and this new one were sent to the
> > stack.
> Just my two cents, if it is this case, you can try to call
> skb_copy_expand on every SKB netback receives to manually create SKBs
> with ~64KB linear buffer to see how it goes...
I've tried it, and it did break everything in a similar way, so that's a 
strong clue that the problem lies here. I've rewrote that part of my 
patches to do less modification, based on Malcolm's idea: netback pulls 
the first frag into linear buffer, then moves a frag from the frag_list 
skb into the first one. That seems to help, but so far I have only one 
relevant test result, I'm waiting for more results.
Zoli


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