Re: [Qemu-devel] Tracing: outstanding tasks

[Prerna Saxena <prerna@linux.vnet.ibm.com>]

On 06/26/2010 01:36 PM, Stefan Hajnoczi wrote:
> Here are the outstanding tasks for QEMU tracing, which Prerna and I have
> been working on.  Tracing aids debugging, profiling, and observing
> execution via lightweight logging at key points in the code path.
>
> The current prototype is available from the 'tracing' branch at:
>
> http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/tracing
>
> This email is both to help Prerna and me focus our efforts, as well as a
> roadmap for the QEMU community to discuss and comment on.
>
> Here are the outstanding tasks for a tracing patchset that can be
> proposed for merge:
>
> 1. Integration with QMP
>
> Owner: Prerna
>
> Currently the trace commands are available from the monitor but a QMP interface
> is needed.
>

Agree. I'd suggest we let this be a TODO till the trace infrastructure 
makes its way upstream.

> 2. More tracepoints need to be added for instrumenting other qemu components
> such as virtio drivers, etc.
>
> Owner: ?
>
> QEMU must come with a useful set of trace events that allows people to get up
> and running quickly.  Focus areas include:
>
>   * guest device emulation
>   * host devices
>   * lifecycle and runloop
>   * memory management
>   * live migration
>
> 3. Documentation
>
> Owner: Stefan
>
> User documentation that explains how to collect traces and add new trace
> events.  I have committed documentation to the tracing branch here:
>
> http://repo.or.cz/w/qemu/stefanha.git/blob_plain/91fde34dc6bfe01af6d5e9265f6a81535d6add15:/docs/tracing.txt
>
> 4. Fix i386-linux-user build
>
> Owner: Prerna
>
>    LINK  i386-linux-user/qemu-i386
> ../simpletrace.o: In function `do_info_all_trace_events':
> /home/stefanha/qemu/simpletrace.c:88: undefined reference to `monitor_printf'
> ../simpletrace.o: In function `do_info_trace':
> /home/stefanha/qemu/simpletrace.c:77: undefined reference to `monitor_printf'
>

I'm looking into this.

> 5. Out-of-line trace file write-out
>
> Owner: Stefan
>
> Trace buffers are written out to file synchronously.  The vcpu thread should
> not be blocked so an async write-out mechanism is needed.
>
> 6. Trace file command
>
> Owner: ?
>
> Traces are written out to hardcoded /tmp/trace.log.  This must be configurable.
> Tracing at startup time should still be possible so configuration needs to
> happen early.

Agree, it is a good-to-have feature.

>
> 7. Binary trace format finalization
>
> Owner: Stefan
>
> We should leave room for extension.  I suggest partitioning the Event ID
> namespace into normal events and special events.  The __trace_begin special
> event is defined to contain the file format version and/or trace record size in
> bytes as the first trace record in the file.
>
> This way, post-processing tools can check the format of the binary trace file.
>
> 8. QMP/monitor command review
>
> Owner: Prerna, Stefan
>
>

9. In its present format, the tracing infrastructure causes at least two 
function calls even for trace events that are disabled. Ideally, there 
should be minimal performance overhead for disabled trace events.
Investigating scope for further optimization.

Owner : Prerna

Thanks,
-- 
Prerna Saxena

Linux Technology Centre,
IBM Systems and Technology Lab,
Bangalore, India