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SubjectRe: TG3 data corruption (TSO ?)
Date
From
Copying netdev.

Benjamin Herrenschmidt wrote:
> Hi !
>
> I've been chasing with Segher a data corruption problem lately.
> Basically transferring huge amount of data (several Gb) and I get
> corrupted data at the rx side. I cannot tell for sure wether what I've
> been observing here is the same problem that segher's been seing on is
> blades, he will confirm or not. He also seemed to imply that reverting
> to an older kernel on the -receiver- side fixed it, which makes me
> wonder, since it's looks really like a sending side problem (see
> explanation below), if some change in, for exmaple, window scaling,
> might hide or trigger it.

Please send me lspci and tg3 probing output so that I know what
tg3 hardware you're using. I also want to look at the tcpdump or
ethereal on the mirrored port that shows the packet being corrupted.

>
> Now, first, I've been playing with ssh from /dev/zero on one machine
> to /dev/zero on the other. That allowed me to run enough
> tests all over
> the place to have some idea of where the problem comes from since ssh
> will shoke at decryption when hitting the corruption.
>
> The base setup where it happens often is 2 Quad G5's connected to a
> gigabit switch. Both were running some versions of 2.6.18-rc4 and -rc5
> (some random git actually, but see below as I've reproduced
> the problem
> with today's git snapshot which includes the TG3 tx race fix among
> others).
>
> I have reproduced with various machines as the receiver. A sungem in a
> Dual G5 and a virtual ethernet in a Power5 partition (so the
> packets go
> to an e1000 then routed through an AIX IO server to a virtual
> ethernet :) are good examples of "variety" :) I haven't tested with
> non-PowerPC machines so far. I've also never been able to
> reproduce with
> TSO disabled on the emitting TG3's
>
> Then, I've hacked tridge "socklib" test program (a simple TCP server
> that pushes a known buffer and a simple TCP receiver that
> connects to it
> and reads the data). I've added comparison of the data with what they
> are supposed to be on the receiving end. The interesting thing is that
> is much faster than ssh or whatever else I tried. ssh or rsync between
> those 2 Quad G5s give me about 35Mb/sec while I get to
> 107Mb/sec average
> with the small test program.
>
> The fun thing is, I've not been able to reproduce at all that
> way. When
> the link is pretty much saturated, the problem doesn't occur !
>
> As soon as I introduce a small delay (some crap waiting loop) in the
> sender to slow down the throughput to about 80Mb/sec, then the problem
> starts occuring every now and then (I don't have precise frquency data
> but I get a corruption every couple of gigabytes I'd say).
>
> As for my previous tests, disabling TSO on the sending side
> "fixes" it.
>
> Below is a dump of what the corruption look like. I've trimmed the
> beginning and end of the dumped packet (the receiver does 8k
> reads). The
> 0x5a are the expected data, the rest is corruption. They look like
> kernel pointers, but that isn't always the case (often though but that
> might not be relevant). The size and position within the buffer of the
> corrupted data is variable (doesn't seem to be specifically a page or
> anything nice and round like that).
>
> I've configured the switch to send all the traffic between the two
> machines to a 3rd box and then recorded it with tcpdump (the
> "spy" uses
> an e1000) and I can see the corrupted data in the recorded
> traces (the exact same pattern as detected by the receiver).
> So it seems
> very likely at this point that the corruption happens on the sending
> side. The TCP checksums are correct I assume. I don't see any error
> count on the receiving tg3 nor suspicous message in dmesg indicating
> they aren't.
>
> That's all the data I have at this point. I can't guarantee 100% that
> it's a TSO bug (it might be a bug that TSO renders visible
> due to timing
> effects) but it looks like it since I've not reproduced yet with TSO
> disabled. I'll do an overnight test to confirm that though...
> sometimes
> the bug can take it's time to show up ... I've seen it wait
> 20Gb before
> it kicked in. Also the fact that fully loading the machine never
> produced it is strange.... smells like a race.
>
> Cheers,
> Ben.
>
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>


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