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Subjectigb and bnx2: "NETDEV WATCHDOG: transmit queue timed out" when skb has huge linear buffer
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.
I suspect that this is the problem as it only happens when guests send
too much slots. Does anyone familiar with these drivers have seen such
issue before? (when these kind of skb's get stucked in the queue)

Regards,

Zoltan Kiss


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