Driver patch to look at...
Randall Stewart
rrs at lakerest.net
Tue Feb 5 11:49:42 UTC 2013
John:
Here is an updated patch, per your suggestions. Note that I also
expanded and the only driver that uses these methods I did not touch
is the cxgb, but thats because I am not really sure it has the problem… it
does not quite enqueue the same way (it appears) that the other drivers do ;-)
R
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On Feb 5, 2013, at 5:49 AM, Randy Stewart wrote:
>
> On Feb 4, 2013, at 3:24 PM, John Baldwin wrote:
>
>> On Monday, February 04, 2013 12:22:49 pm Randy Stewart wrote:
>>> All:
>>>
>>> I have been working with TCP in gigabit networks (igb driver actually) and have
>>> found a very nasty problem with the way the driver is doing its put back when
>>> it fills the out-bound transmit queue.
>>>
>>> Basically it has taken a packet from the head of the ring buffer, and then
>>> realizes it can't fit it into the transmit queue. So it just re-enqueue's it
>>> into the ring buffer. Whats wrong with that? Well most of the time there
>>> are anywhere from 10-50 packets (maybe more) in that ring buffer when you are
>>> operating at full speed (or trying to). This means you will see 10 duplicate
>>> ACKs, do a fast retransmit and cut your cwnd in half.. not very nice actually.
>>>
>>> The patch I have attached makes it so that
>>>
>>> 1) There are ways to swap back.
>>> 2) Use the peek in the ring buffer and only
>>> dequeue the packet if we put it into the transmit ring
>>> 3) If something goes wrong and the transmit frees the packet we dequeue it.
>>> 4) If the transmit changed it (defrag etc) then swap out the new mbuf that
>>> has taken its place.
>>>
>>> I have fixed the four intel drivers that had this systemic issue, but there
>>> are still more to fix.
>>>
>>> Comments/review .. rotten egg's etc.. would be most welcome before
>>> I commit this..
>>
>> Does this only happen in drivers that use buffering?
>
> Yep, there are a lot of drivers that *do not* use the drbr_xxxx() functions and
> for those they do the IFQ_DRV_PREPEND().. its only the newer drivers like the
> intel 1Gig and 10Gig ones that seem effected
>
> Also effected are :
>
> bxe
> cxgb
> oce
> en
>
> I have not fixed those yet.
>
>> I seem to recall that
>> drivers using IFQ would just stuff the packet at the head of the IFQ via
>> IFQ_DRV_PREPEND() in this case so it is still the next packet to transmit.
>> See, for example, this bit in dc_start_locked():
>>
>> for (queued = 0; !IFQ_DRV_IS_EMPTY(&ifp->if_snd); ) {
>> /*
>> * If there's no way we can send any packets, return now.
>> */
>> if (sc->dc_cdata.dc_tx_cnt > DC_TX_LIST_CNT - DC_TX_LIST_RSVD) {
>> ifp->if_drv_flags |= IFF_DRV_OACTIVE;
>> break;
>> }
>> IFQ_DRV_DEQUEUE(&ifp->if_snd, m_head);
>> if (m_head == NULL)
>> break;
>>
>> if (dc_encap(sc, &m_head)) {
>> if (m_head == NULL)
>> break;
>> IFQ_DRV_PREPEND(&ifp->if_snd, m_head);
>> ifp->if_drv_flags |= IFF_DRV_OACTIVE;
>> break;
>> }
>>
>> It sounds like drbr/buf_ring just don't handle this case correctly? It
>> seems a shame to have to duplicate so much code in the various drivers to
>> fix this, but that seems to be par for the course when using buf_ring. :(
>> (buggy in edge cases and lots of duplicated code that is).
>
>> Also, doing the drbr_swap() just so that drbr_dequeue() returns what you
>> just swapped in seems... odd. It seems that it would be nicer instead
>> to have some sort of drbr_peek() / drbr_advance() where the latter just
>> skips over whatever the current head is? Then you could have something
>> like:
>>
>> while ((next = drbr_peek()) != NULL) {
>> if (!foo_encap(&next)) {
>> if (next == NULL)
>> drbr_advance();
>> break;
>> }
>> drbr_advance();
>> }
>>
>
> That was what I originally did (without the rename), but there is a sure crash waiting in that.
> The only difference from what I originally had was just drbr_dequeue().. but
> I was being a bit lazy and not wanting to add yet another function to the
> drbr_xxxx code since essential it would be a clone of drbr_dequeue() without
> returning the mbuf.
>
> The crash potential here is in that foo_encap(&next) often times will return
> a different mbuf (at least in the igb driver it does). I believe its due
> to either the m_pullup() which could change the lead mbuf you want
> in the drbr_ring, or the m_defrag all within igb_xmit. Thus you have
> to track what comes back from the !foo_encap() call and compare it to
> see if you must swap it out.
>
>
>> I guess the sticky widget is the case of ATLQ as you need to dequeue the
>> packet always in the ALTQ case and put it back if the encap fails.
>
> Yeah ALTQ is ugly and IMO we need to re-write it anyway.. maybe re-think
> this whole layer ;-0
>
>> For
>> your patch it's not clear to me how that works. It seems that if the
>> encap routine frees the mbuf you try to dereference a freed pointer when
>> you call drbr_dequeue().
>
> Hmm you are right.. I forgot how we keep those using the mbuf itself...
>
>> I really think you will instead need some sort
>> of 'drbr_putback()' and have 'drbr_peek()' dequeue in the ALTQ case and
>> use 'drbr_putback()' to put it back (PREPEND) in the ALTQ case.
>
> We could do that but drbr_putback() would probably need both the old
> and new pointers and then we could make it do the ring_swap() to put
> the right mbuf in place..
>
> Let me go explore that and come up with a better patch ;-)
>
> R
>
>
>>
>> --
>> John Baldwin
>> _______________________________________________
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>>
>
> -----
> Randall Stewart
> randall at lakerest.net
>
>
>
>
> _______________________________________________
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> http://lists.freebsd.org/mailman/listinfo/freebsd-net
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>
------------------------------
Randall Stewart
803-317-4952 (cell)
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