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

[Yuchung Cheng]

On Tue, Sep 26, 2017 at 5:12 PM, Yuchung Cheng <ycheng@xxxxxxxxxx> wrote:
> On Tue, Sep 26, 2017 at 6:10 AM, Roman Gushchin <guro@xxxxxx> wrote:
>>> On Wed, Sep 20, 2017 at 6:46 PM, Roman Gushchin <guro@xxxxxx> 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!