Re: [Qemu-devel] Static tracepoint control via trace-event

[Jan Kiszka]

Am 19.10.2010 15:52, Stefan Hajnoczi wrote:
> On Tue, Oct 19, 2010 at 2:36 PM, Daniel P. Berrange <address@hidden> wrote:
>> On Tue, Oct 19, 2010 at 03:08:08PM +0200, Jan Kiszka wrote:
>>> Hi Stefan,
>>>
>>> just had a closer look at qemu's new tracing framework. Looks cool,
>>> though it leaves a bit room for improvements. ;)
>>>
>>> One quirk I stumbled over quickly was the "disable" tag in trace-events.
>>> It confused me first as qemu starts without any tracepoint enabled by
>>> default and I thought I had to hack the file. Then I read the doc and
>>> wondered which exiting or future backend would come without sufficiently
>>> fast dynamic tracepoint control. Do you have any in mind?
>>>
>>> Instead of making it a compile-time switch (except for simpletrace), I
>>> would vote for declaring the simpletrace usage as the only one: disable
>>> sets the default state of the dynamic tracepoint. That way we could use
>>> trace-events to define a useful set of standard, moderate-impact
>>> tracepoints that shall be on. Others will still be available once a
>>> backend is configured, but remain off until enabled during runtime.
>>> Anything else looks like overkill to me.
>>
>> FYI with the DTrace/SystemTAP backend I posted yesterday, the 'disable'
>> keyword is effectively completely ignored. All tracepoints are disabled
>> when QEMU is running normally. Only when a end user runs a dtrace script
>> that references a QEMU tracepoint, is that specific tracepoint enabled.
> 
> I think that makes sense for external trace backends.  DTrace can
> launch a process for you with the probes you want enabled from the
> start.  The simpletrace backend can't really do this so probes can be
> enabled/disabled at compile-time (e.g. early startup tracing).

Once we have "-trace events=...", defining the list of active
tracepoints before starting qemu will be trivial (e.g. via a config
file). Of course, this requires that all tracepoints are built-in...

Jan

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