Re: [PATCH RFC 0/4]Porting Xentrace to kvm

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> Jan Kiszka wrote:
>> Avi Kivity wrote:
>>  
>>> Liu, Eric E wrote:
>>>    
>>>> Hi,
>>>> The following patches port xentrace to kvm which is useful for
>>>> performance tuning and debugging.
>>>>
>>>> It is designed to allow debugging traces of kvm to be generated on
>>>> Up/Smp machines. Each trace entry is outputted in a trace ring buffer
>>>> for per cpu which is mapped to userspace, and the userspace tools can
>>>> analyze the data according to some formats definitions.
>>>>
>>>> Since we already have had debugfs_entries and some other kernel debug
>>>> mechanism to use, does this kvmtrace make sense? Any comment is
>>>> welcomed.         
>>> This looks very useful.  While kvm_stat provides good data, this is
>>> much more in depth.
>>>
>>> The kernel already contains a method of transferring percpu data to
>>> userspace; see Documentation/filesystems/relay.txt.  It supports
>>> mmap() and read().  Please see if it is a good fit.
>>>     
>>
>> I would also suggest to use the new kernel standard for instrumentation:
>> trace_mark().
>>
>> This would also allow to reuse the trace points with other tracer and
>> maybe even obsolete a separate transportation channel, e.g. when LTTng
>> is once :-/ merged.
>>   
> 
> Can one have markers automatically recorded?  Or do you need to connect
> the marker with a logging function?

Yes, of course you need a probe function. Either your own one, or one
provided by other tracers.

> 
> If the latter, it's too difficult to use.

I don't think it is. Check linux/samples/markers/probe-example.c. BTW,
RCU-Trace (not yet mainlined) make use of markers as well, see -rt.

And in contrast to the suggested implementation, markers have less
impact on the code if built-in but disabled. So there would be no
reasons for not shipping kvm production binaries that are prepared to be
traced.

But if you prefer the inlined version, I bet you'll have to hide it from
reviewers at LKML. ;)

Jan


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