Re: [PATCH] clocksource, Add warning to clocksource_delta() validation code

[John Stultz]

On Fri, Oct 17, 2014 at 11:23 AM, Prarit Bhargava <prarit@xxxxxxxxxx> wrote:
>
>
> On 10/17/2014 02:17 PM, John Stultz wrote:
>> On Fri, Oct 17, 2014 at 6:57 AM, Prarit Bhargava <prarit@xxxxxxxxxx> wrote:
>>> A bug report came in against an older kernel which output "backward time"
>>> messages and the report noted that the upstream kernel worked.  After some
>>> investigation it turned out that one of the sockets was bad on the system
>>> and the "backward time" messages were caused by a real, but intermittent,
>>> hardware failure.
>>>
>>> Commit 09ec54429c6d10f87d1f084de53ae2c1c3a81108 ("clocksource: Move
>>> cycle_last validation to core code") modifies the x86 clocksource such that
>>> if a negative delta between two reads of time is calculated the
>>> clocksource_delta() code will return 0.  There is no warning when this
>>> occurs and there really should be one in order to catch not only hardware
>>> issues like the issue above, but potential coding issues as the code is
>>> modified.  This patch introduces a WARN() which will also dump a stack
>>> trace to the console so the exact code path can be evaluated.
>>>
>>> I tested this by booting on the broken hardware and left the system idle
>>> until a negative clocksource_delta() event occurred.
>>>
>>> Cc: John Stultz <john.stultz@xxxxxxxxxx>
>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>> Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
>>> ---
>>>  kernel/time/timekeeping_internal.h |    7 ++++++-
>>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h
>>> index 4ea005a..abe6bc8 100644
>>> --- a/kernel/time/timekeeping_internal.h
>>> +++ b/kernel/time/timekeeping_internal.h
>>> @@ -17,7 +17,12 @@ static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)
>>>  {
>>>         cycle_t ret = (now - last) & mask;
>>>
>>> -       return (s64) ret > 0 ? ret : 0;
>>> +       if ((s64)ret > 0)
>>> +               return ret;
>>> +
>>> +       WARN(1, "Clocksource calculated negative delta, %lld.  last = %llu, now = %llu, mask = %llx\n",
>>> +            (s64)ret, last, now, mask);
>>> +       return 0;
>>
>>
>> I realize you followed up that this wasn't finished, but just as some
>> feedback, there's a number of types of hardware where there may be a
>> very slight skew between cpu TSC, and this will briefly trigger right
>> after each timekeeping update if a system is reading the clock
>> frequently (think of the case where the update happens on the cpu
>> thats just a little bit ahead, while a timestamping loop is running on
>> a cpu that is a little bit behind).
>
> Ah, interesting.  Okay ... drop this patch then.  Thanks for the info John.

If you're wanting something that aids with debugging, maybe some sort
calmly stated warn-once in the dmesg might be ok, that or some other
flag exported via a debugging interface.

thanks
-john
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