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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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...

Wei.

> 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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