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

[Yuchung Cheng]

On Thu, Sep 28, 2017 at 1:14 AM, Oleksandr Natalenko
<oleksandr@xxxxxxxxxxxxxx> 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 stÅeda 27. zÃÅÃ 2017 2:18:32 CEST Yuchung Cheng wrote:
>> 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.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!
>
>