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    SubjectRe: [bug] stuck localhost TCP connections, v2.6.26-rc3+

    * Ingo Molnar <> wrote:

    > in an overnight -tip testruns that is based on recent -git i got two
    > stuck TCP connections:
    > Active Internet connections (w/o servers)
    > Proto Recv-Q Send-Q Local Address Foreign Address State
    > tcp 0 174592 ESTABLISHED
    > tcp 72134 0 ESTABLISHED

    update: in the past 5 days of -tip testing i've gathered about 10
    randconfig kernel configs that all produced such failures.

    Since the bug itself is very elusive (it takes up to 50 boot +
    kernel-rebuild-via-distccc iterations to trigger) bisection was still
    not an option - but with 10 configs statistical analysis of the configs
    is now possible.

    I made a histogram of all kernel options present in those configs, and
    one networking related kernel option stood out:


    that code is called in the bootlogs:

    > [ 13.279410] calling cubictcp_register+0x0/0x80
    > [ 13.279412] TCP cubic registered

    the likelyhood of CONFIG_TCP_CONG_CUBIC=y being enabled in my randconfig
    runs is 75%. The likelyhood of CONFIG_TCP_CONG_CUBIC=y being enabled in
    10 configs in a row is 0.75^10, or 5.6%. So statistical analysis can say
    it with a 95% confidence that the presence of this option correlates to
    the hung sockets.

    i have started testing this theory now, via the patch below, which turns
    off TCP_CONG_CUBIC. It will take about 50 bootups on the affected
    testsystems to confirm. (it will take a couple of hours today as not all
    testsystems show these hung socket symptoms)

    distributions enable TCP_CONG_CUBIC by default:

    $ grep CUBIC /boot/config-

    which would explain why Arjan and Peter triggered similar hangs as well.


    Subject: qa: no TCP_CONG_CUBIC
    From: Ingo Molnar <>
    Date: Thu May 29 09:45:51 CEST 2008

    net/ipv4/Kconfig | 4 ++++
    1 file changed, 4 insertions(+)

    Index: tip/net/ipv4/Kconfig
    --- tip.orig/net/ipv4/Kconfig
    +++ tip/net/ipv4/Kconfig
    @@ -454,6 +454,8 @@ config TCP_CONG_BIC
    config TCP_CONG_CUBIC
    tristate "CUBIC TCP"
    default y
    + depends on BROKEN_BOOT_ALLOWED
    + select BROKEN_BOOT
    This is version 2.0 of BIC-TCP which uses a cubic growth function
    among other techniques.
    @@ -608,6 +610,8 @@ endif
    config TCP_CONG_CUBIC
    depends on !TCP_CONG_ADVANCED
    + depends on BROKEN_BOOT_ALLOWED
    + select BROKEN_BOOT
    default y


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