Re: [Xen-devel] [PATCH v5 3/4] xen: refactor debugtrace data

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.09.2019 10:49, Juergen Gross wrote:
> On 05.09.19 16:43, Jan Beulich wrote:
>> On 05.09.2019 16:36, Juergen Gross wrote:
>>> On 05.09.19 14:46, Juergen Gross wrote:
>>>> On 05.09.19 14:37, Jan Beulich wrote:
>>>>> On 05.09.2019 14:27, Juergen Gross wrote:
>>>>>> On 05.09.19 14:22, Jan Beulich wrote:
>>>>>>> On 05.09.2019 14:12, Juergen Gross wrote:
>>>>>>>> On 05.09.19 14:01, Jan Beulich wrote:
>>>>>>>>> On 05.09.2019 13:39, Juergen Gross wrote:
>>>>>>>>>> As a preparation for per-cpu buffers do a little refactoring of the
>>>>>>>>>> debugtrace data: put the needed buffer admin data into the buffer as
>>>>>>>>>> it will be needed for each buffer. In order not to limit buffer size
>>>>>>>>>> switch the related fields from unsigned int to unsigned long, as on
>>>>>>>>>> huge machines with RAM in the TB range it might be interesting to
>>>>>>>>>> support buffers >4GB.
>>>>>>>>>
>>>>>>>>> Just as a further remark in this regard:
>>>>>>>>>
>>>>>>>>>>      void debugtrace_printk(const char *fmt, ...)
>>>>>>>>>>      {
>>>>>>>>>>          static char buf[DEBUG_TRACE_ENTRY_SIZE];
>>>>>>>>>> -    static unsigned int count, last_count, last_prd;
>>>>>>>>>> +    static unsigned int count, last_count;
>>>>>>>>>
>>>>>>>>> How long do we think will it take until their wrapping will become
>>>>>>>>> an issue with such huge buffers?
>>>>>>>>
>>>>>>>> Count wrapping will not result in any misbehavior of tracing. With
>>>>>>>> per-cpu buffers it might result in ambiguity regarding sorting the
>>>>>>>> entries, but I guess chances are rather low this being a real issue.
>>>>>>>>
>>>>>>>> BTW: wrapping of count is not related to buffer size, but to the
>>>>>>>> amount of trace data written.
>>>>>>>
>>>>>>> Sure, but the chance of ambiguity increases with larger buffer sizes.
>>>>>>
>>>>>> Well, better safe than sorry. Switching to unsigned long will rarely
>>>>>> hurt, so I'm going to do just that. The only downside will be some waste
>>>>>> of buffer space for very long running traces with huge amounts of
>>>>>> entries.
>>>>>
>>>>> Hmm, true. Maybe we could get someone else's opinion on this - anyone?
>>>>
>>>> TBH, I wouldn't be concerned about the buffer space. In case there are
>>>> really billions of entries, the difference between a 10-digit count
>>>> value and maybe a 15 digit one (and that is already a massive amount)
>>>> isn't that large. The average printed size of count with about
>>>> 4 billion entries is 9.75 digits (and not just 5 :-) ).
>>>
>>> Another option would be to let count wrap at e.g. 4 billion (or even 1
>>> million) and add a wrap count. Each buffer struct would contain the
>>> wrap count of the last entry, and when hitting a higher wrap count a
>>> new entry like ("wrap %u->%u", old_wrap, new_wrap) would be printed.
>>> This saves buffer space without loosing information.
>>
>> This sounds quite neat.
> 
> I'm adding a new patch for that purpose, as it is adding a new feature.
> 
> Any preferences for the max value of count? I'm in favor of limiting it
> to 6-digit numbers as those are much easier to compare by just looking
> at them.

I'm not overly fussed; personally I'd probably wrap at 30 bits, making it
generally up to 8 digits, just very slightly going into the 9-digit range.

Jan

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