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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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