When CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is set sched_clock() gets the
time from hardware, such as from TSC.  In this configuration kgdb will
report a softlock warning messages 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 "get_timestamp(this_cpu)"

Why "touch_timestamp" is 100 sec, instead of 180 sec?

With the CONFIG_HAVE_UNSTABLE_SCHED_CLOCK" set the call trace of
get_timestamp() is:

get_timestamp(this_cpu) -->cpu_clock(this_cpu)
-->sched_clock_cpu(this_cpu) -->__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" values 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(), which allows kgdb to
request that the sched clock is updated when the watchdog thread runs
the first time after a resume from kgdb.
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Dongdong Deng <Dongdong.Deng@windriver.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: peterz@infradead.org