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Date:	Sat, 26 Sep 2009 11:01:37 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jason Wessel <jason.wessel@...driver.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"Deng, Dongdong" <Dongdong.Deng@...driver.com>
Subject: Re: [PATCH] softlockup: fix problem with long kernel pauses from kgdb

On Tue, Aug 4, 2009 at 10:16 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Jason Wessel <jason.wessel@...driver.com> wrote:
>
>> Peter Zijlstra wrote:
>> > On Mon, 2009-07-27 at 15:03 -0500, Jason Wessel wrote:
>> >> The fix is to simply invoke sched_clock_tick() to update "cpu sched
>> >> clock" on exit from kgdb_handle_exception.
>> >
>> > Is that a regular IRQ context, or is that NMI context?
>> >
>> >> Signed-off-by: Dongdong Deng <Dongdong.Deng@...driver.com>
>> >> Signed-off-by: Jason Wessel <jason.wessel@...driver.com>
>> >> Cc: Ingo Molnar <mingo@...e.hu>
>> >> Cc: peterz@...radead.org
>> >> ---
>> >>  kernel/softlockup.c |    3 +++
>> >>  1 file changed, 3 insertions(+)
>> >>
>> >> --- a/kernel/softlockup.c
>> >> +++ b/kernel/softlockup.c
>> >> @@ -118,6 +118,9 @@ void softlockup_tick(void)
>> >>         }
>> >>
>> >>         if (touch_timestamp == 0) {
>> >> +               /* If the time stamp was touched externally make sure the
>> >> +                * scheduler tick is up to date as well */
>> >> +               sched_clock_tick();
>> >>                 __touch_softlockup_watchdog();
>> >>                 return;
>> >>         }
>> >>
>> >
>> > Aside from the funny comment style (please fix) the fix does look
>> > sensible.
>>
>> It turns out that further testing of this patch shows a regression in
>> the ability to detect certain lockups.  It is a direct result of the
>> way the scheduling code makes use of the touch_softlockup_watchdog().
>> With the above proposed patch the tick was getting updated after a
>> resume, but was also getting updated with the run_timers(), and if
>> that happened before the softlockup tick, no softlockup would get
>> reported (note that I was using some test code to induce softlockups).
>>
>> The patch below is a bit more invasive but solves the problem by
>> allowing kgdb to request that the sched cpu clock is updated only when
>> returning from a state where we know we need to force the update.
>>
>> Would this change be an acceptable solution to allow kgdb to
>> peacefully exist with the softlockup code?
>>
>> Thanks,
>> Jason.
>>
>>
>> -----
>> From: Jason Wessel <jason.wessel@...driver.com>
>> Subject: [PATCH] softlockup: add sched_clock_tick() to avoid kernel warning on kgdb resume
>>
>> 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@...driver.com>
>> Signed-off-by: Dongdong Deng <Dongdong.Deng@...driver.com>
>> Cc: Ingo Molnar <mingo@...e.hu>
>> Cc: peterz@...radead.org
>>
>> ---
>>  include/linux/sched.h |    4 ++++
>>  kernel/kgdb.c         |    6 +++---
>>  kernel/softlockup.c   |   16 ++++++++++++++++
>>  3 files changed, 23 insertions(+), 3 deletions(-)
>>
>> --- a/kernel/softlockup.c
>> +++ b/kernel/softlockup.c
>> @@ -79,6 +79,14 @@ void touch_softlockup_watchdog(void)
>>  }
>>  EXPORT_SYMBOL(touch_softlockup_watchdog);
>>
>> +static int softlock_touch_sync[NR_CPUS];
>> +
>> +void touch_softlockup_watchdog_sync(void)
>> +{
>> +     softlock_touch_sync[raw_smp_processor_id()] = 1;
>> +     __raw_get_cpu_var(touch_timestamp) = 0;
>> +}
>> +
>>  void touch_all_softlockup_watchdogs(void)
>>  {
>>       int cpu;
>> @@ -118,6 +126,14 @@ void softlockup_tick(void)
>>       }
>>
>>       if (touch_timestamp == 0) {
>> +             if (unlikely(softlock_touch_sync[this_cpu])) {
>> +                     /*
>> +                      * If the time stamp was touched atomically
>> +                      * make sure the scheduler tick is up to date.
>> +                      */
>> +                     softlock_touch_sync[this_cpu] = 0;
>> +                     sched_clock_tick();
>> +             }
>
> Hm, this looks quite ugly. Peter, Thomas, can you think of a cleaner
> solution?
>

We need not to sync up sched_clock in softlockup_tick, it can just return
and sync up the timestamp in the next tick. This will not touch what should
be done by dynticks, scheduler time and so on and keep softlockup_tick
clean.

I have rewrote the patch based on above, please check the attachment.

Thanks,
Yong

>        Ingo
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View attachment "0001-softlockup-introduce-touch_softlockup_watchdog_sync.patch" of type "text/x-patch" (3305 bytes)

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