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SubjectRe: [REGRESSION] Warning in tcp_fastretrans_alert() of net/ipv4/tcp_input.c
On Thu, Sep 28, 2017 at 1:14 AM, Oleksandr Natalenko
<oleksandr@natalenko.name> wrote:
> 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.

Hi Oleksandr: no retrans_collapse should not matter for that warning
in tcp_fstretrans_alert(). the warning as I explained earlier is
likely false. Neal and I are more concerned the panic in
tcp_sacktag_walk. This is just a blind shot but thx for retrying.

We can submit a one-liner to remove the fast retrans warning but want
to nail the bigger issue first.

>
> 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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