• Jason Wessel's avatar
    softlockup: Add sched_clock_tick() to avoid kernel warning on kgdb resume · d6ad3e28
    Jason Wessel authored
    When CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is set, sched_clock() gets
    the time from hardware such as the TSC on x86. In this
    configuration kgdb will report a softlock warning message on
    resuming or detaching from a debug session.
    Sequence of events in the problem case:
     1) "cpu sched clock" and "hardware time" are at 100 sec prior
        to a call to kgdb_handle_exception()
     2) Debugger waits in kgdb_handle_exception() for 80 sec and on
        exit the following is called ...  touch_softlockup_watchdog() -->
        __raw_get_cpu_var(touch_timestamp) = 0;
     3) "cpu sched clock" = 100s (it was not updated, because the
        interrupt was disabled in kgdb) but the "hardware time" = 180 sec
     4) The first timer interrupt after resuming from
        kgdb_handle_exception updates the watchdog from the "cpu sched clock"
    update_process_times() { ...  run_local_timers() -->
    softlockup_tick() --> check (touch_timestamp == 0) (it is "YES"
    here, we have set "touch_timestamp = 0" at kgdb) -->
    __touch_softlockup_watchdog() ***(A)--> reset "touch_timestamp"
    to "get_timestamp()" (Here, the "touch_timestamp" will still be
    set to 100s.)  ...
        scheduler_tick() ***(B)--> sched_clock_tick() (update "cpu sched
        clock" to "hardware time" = 180s) ...  }
     5) The Second timer interrupt handler appears to have a large
        jump and trips the softlockup warning.
    update_process_times() { ...  run_local_timers() -->
    softlockup_tick() --> "cpu sched clock" - "touch_timestamp" =
    180s-100s > 60s --> printk "soft lockup error messages" ...  }
    note: ***(A) reset "touch_timestamp" to
    Why is "touch_timestamp" 100 sec, instead of 180 sec?
    When CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is set, the call trace of
    get_timestamp() is:
     -->__update_sched_clock(sched_clock_data, now)
    The __update_sched_clock() function uses the GTOD tick value to
    create a window to normalize the "now" values.  So if "now"
    value is too big for sched_clock_data, it will be ignored.
    The fix is to invoke sched_clock_tick() to update "cpu sched
    clock" in order to recover from this state.  This is done by
    introducing the function touch_softlockup_watchdog_sync(). This
    allows kgdb to request that the sched clock is updated when the
    watchdog thread runs the first time after a resume from kgdb.
    [yong.zhang0@gmail.com: Use per cpu instead of an array]
    Signed-off-by: default avatarJason Wessel <jason.wessel@windriver.com>
    Signed-off-by: default avatarDongdong Deng <Dongdong.Deng@windriver.com>
    Cc: kgdb-bugreport@lists.sourceforge.net
    Cc: peterz@infradead.org
    LKML-Reference: <1264631124-4837-2-git-send-email-jason.wessel@windriver.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
softlockup.c 7.04 KB