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SubjectRe: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp_input.c
On Tue, Sep 26, 2017 at 5:12 PM, Yuchung Cheng <ycheng@google.com> wrote:
> On Tue, Sep 26, 2017 at 6:10 AM, Roman Gushchin <guro@fb.com> wrote:
>>> On Wed, Sep 20, 2017 at 6:46 PM, Roman Gushchin <guro@fb.com> wrote:
>>> >
>>> > > Hello.
>>> > >
>>> > > Since, IIRC, v4.11, there is some regression in TCP stack resulting in the
>>> > > warning shown below. Most of the time it is harmless, but rarely it just
>>> > > causes either freeze or (I believe, this is related too) panic in
>>> > > tcp_sacktag_walk() (because sk_buff passed to this function is NULL).
>>> > > Unfortunately, I still do not have proper stacktrace from panic, but will try
>>> > > to capture it if possible.
>>> > >
>>> > > Also, I have custom settings regarding TCP stack, shown below as well. ifb is
>>> > > used to shape traffic with tc.
>>> > >
>>> > > Please note this regression was already reported as BZ [1] and as a letter to
>>> > > ML [2], but got neither attention nor resolution. It is reproducible for (not
>>> > > only) me on my home router since v4.11 till v4.13.1 incl.
>>> > >
>>> > > Please advise on how to deal with it. I'll provide any additional info if
>>> > > necessary, also ready to test patches if any.
>>> > >
>>> > > Thanks.
>>> > >
>>> > > [1] https://bugzilla.kernel.org/show_bug.cgi?id=195835
>>> > > [2] https://urldefense.proofpoint.com/v2/url?u=https-3A__www.spinics.net_lists_netdev_msg436158.html&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=jJYgtDM7QT-W-Fz_d29HYQ&m=MDDRfLG5DvdOeniMpaZDJI8ulKQ6PQ6OX_1YtRsiTMA&s=-n3dGZw-pQ95kMBUfq5G9nYZFcuWtbTDlYFkcvQPoKc&e=
>>> >
>>> > We're experiencing the same problems on some machines in our fleet.
>>> > Exactly the same symptoms: tcp_fastretrans_alert() warnings and
>>> > sometimes panics in tcp_sacktag_walk().
>>> >
>>> > Here is an example of a backtrace with the panic log:
>>
>> Hi Yuchung!
>>
>>> do you still see the panics if you disable RACK?
>>> sysctl net.ipv4.tcp_recovery=0?
>>
>> No, we haven't seen any crash since that.
> I am out of ideas how RACK can potentially cause tcp_sacktag_walk to
> take an empty skb :-( Do you have stack trace or any hint on which call
> to tcp-sacktag_walk triggered the panic? internally at Google we never
> see that.
hmm something just struck me: could you try
sysctl net.ipv4.tcp_recovery=1 net.ipv4.tcp_retrans_collapse=0
and see if kernel still panics on sack processing?

>
>
>>
>>>
>>> also have you experience any sack reneg? could you post the output of
>>> ' nstat |grep -i TCP' thanks
>>
>> hostname TcpActiveOpens 2289680 0.0
>> hostname TcpPassiveOpens 3592758 0.0
>> hostname TcpAttemptFails 746910 0.0
>> hostname TcpEstabResets 154988 0.0
>> hostname TcpInSegs 16258678255 0.0
>> hostname TcpOutSegs 46967011611 0.0
>> hostname TcpRetransSegs 13724310 0.0
>> hostname TcpInErrs 2 0.0
>> hostname TcpOutRsts 9418798 0.0
>> hostname TcpExtEmbryonicRsts 2303 0.0
>> hostname TcpExtPruneCalled 90192 0.0
>> hostname TcpExtOfoPruned 57274 0.0
>> hostname TcpExtOutOfWindowIcmps 3 0.0
>> hostname TcpExtTW 1164705 0.0
>> hostname TcpExtTWRecycled 2 0.0
>> hostname TcpExtPAWSEstab 159 0.0
>> hostname TcpExtDelayedACKs 209207209 0.0
>> hostname TcpExtDelayedACKLocked 508571 0.0
>> hostname TcpExtDelayedACKLost 1713248 0.0
>> hostname TcpExtListenOverflows 625 0.0
>> hostname TcpExtListenDrops 625 0.0
>> hostname TcpExtTCPHPHits 9341188489 0.0
>> hostname TcpExtTCPPureAcks 1434646465 0.0
>> hostname TcpExtTCPHPAcks 5733614672 0.0
>> hostname TcpExtTCPSackRecovery 3261698 0.0
>> hostname TcpExtTCPSACKReneging 12203 0.0
>> hostname TcpExtTCPSACKReorder 433189 0.0
>> hostname TcpExtTCPTSReorder 22694 0.0
>> hostname TcpExtTCPFullUndo 45092 0.0
>> hostname TcpExtTCPPartialUndo 22016 0.0
>> hostname TcpExtTCPLossUndo 2150040 0.0
>> hostname TcpExtTCPLostRetransmit 60119 0.0
>> hostname TcpExtTCPSackFailures 2626782 0.0
>> hostname TcpExtTCPLossFailures 182999 0.0
>> hostname TcpExtTCPFastRetrans 4334275 0.0
>> hostname TcpExtTCPSlowStartRetrans 3453348 0.0
>> hostname TcpExtTCPTimeouts 1070997 0.0
>> hostname TcpExtTCPLossProbes 2633545 0.0
>> hostname TcpExtTCPLossProbeRecovery 941647 0.0
>> hostname TcpExtTCPSackRecoveryFail 336302 0.0
>> hostname TcpExtTCPRcvCollapsed 461354 0.0
>> hostname TcpExtTCPAbortOnData 349196 0.0
>> hostname TcpExtTCPAbortOnClose 3395 0.0
>> hostname TcpExtTCPAbortOnTimeout 51201 0.0
>> hostname TcpExtTCPMemoryPressures 2 0.0
>> hostname TcpExtTCPSpuriousRTOs 2120503 0.0
>> hostname TcpExtTCPSackShifted 2613736 0.0
>> hostname TcpExtTCPSackMerged 21358743 0.0
>> hostname TcpExtTCPSackShiftFallback 8769387 0.0
>> hostname TcpExtTCPBacklogDrop 5 0.0
>> hostname TcpExtTCPRetransFail 843 0.0
>> hostname TcpExtTCPRcvCoalesce 949068035 0.0
>> hostname TcpExtTCPOFOQueue 470118 0.0
>> hostname TcpExtTCPOFODrop 9915 0.0
>> hostname TcpExtTCPOFOMerge 9 0.0
>> hostname TcpExtTCPChallengeACK 90 0.0
>> hostname TcpExtTCPSYNChallenge 3 0.0
>> hostname TcpExtTCPFastOpenActive 2089 0.0
>> hostname TcpExtTCPSpuriousRtxHostQueues 896596 0.0
>> hostname TcpExtTCPAutoCorking 547386735 0.0
>> hostname TcpExtTCPFromZeroWindowAdv 28757 0.0
>> hostname TcpExtTCPToZeroWindowAdv 28761 0.0
>> hostname TcpExtTCPWantZeroWindowAdv 322431 0.0
>> hostname TcpExtTCPSynRetrans 3026 0.0
>> hostname TcpExtTCPOrigDataSent 40976870977 0.0
>> hostname TcpExtTCPHystartTrainDetect 453920 0.0
>> hostname TcpExtTCPHystartTrainCwnd 11586273 0.0
>> hostname TcpExtTCPHystartDelayDetect 10943 0.0
>> hostname TcpExtTCPHystartDelayCwnd 763554 0.0
>> hostname TcpExtTCPACKSkippedPAWS 30 0.0
>> hostname TcpExtTCPACKSkippedSeq 218 0.0
>> hostname TcpExtTCPWinProbe 2408 0.0
>> hostname TcpExtTCPKeepAlive 213768 0.0
>> hostname TcpExtTCPMTUPFail 69 0.0
>> hostname TcpExtTCPMTUPSuccess 8811 0.0
>>
>> Thanks!

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