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SubjectRe: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp_input.c
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Hi.

Won't tell about panic in tcp_sacktag_walk() since I cannot trigger it
intentionally, but setting net.ipv4.tcp_retrans_collapse to 0 *does not* fix
warning in tcp_fastretrans_alert() for me.

On středa 27. září 2017 2:18:32 CEST Yuchung Cheng wrote:
> 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.ne
> >>> > > t_lists_netdev_msg436158.html&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=jJ
> >>> > > YgtDM7QT-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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