Re: [PATCH v2 3/3] trace: Example of "centralized" recorder tracing

[Christophe de Dinechin <dinechin@redhat.com>]

On 2020-07-02 at 15:47 CEST, Stefan Hajnoczi wrote...
> On Wed, Jul 01, 2020 at 05:15:01PM +0100, Daniel P. Berrangé wrote:
>> On Wed, Jul 01, 2020 at 05:09:06PM +0100, Stefan Hajnoczi wrote:
>> > On Tue, Jun 30, 2020 at 01:41:36PM +0100, Daniel P. Berrangé wrote:
>> > > On Fri, Jun 26, 2020 at 06:27:06PM +0200, Christophe de Dinechin wrote:
>> > > IMHO the whole point of having the pluggable trace backend impls, is
>> > > precisely that we don't have to add multiple different calls in the
>> > > code. A single trace_qemu_mutex_unlock() is supposed to work with
>> > > any backend.
>> >
>> > I think an exception is okay when the other trace backends do not offer
>> > equivalent functionality.
>> >
>> > Who knows if anyone other than Christophe will use this functionality,
>> > but it doesn't cost much to allow it.
>>
>> This patch is just an example though, suggesting this kind of usage is
>> expected to done in other current trace probe locations. The trace wrapper
>> has most of the information required already including a format string,
>> so I'd think it could be wired up to the generator so we don't add extra
>> record() statements through the codebase.

The primary purpose of the recorder is post-mortem dumps, flight recorder
style. Tracing is only a secondary benefit. Not sure if it's worth making a
distinction between events you want to record and those you want to trace.
(Example: You might want to record all command line options, but almost
never trace them)

> At most it should require an
>> extra annotation in the trace-events file to take the extra parameter
>> for grouping, and other trace backends can ignore that.
>
> It's true, it may be possible to put this functionality in the
> trace-events.

Let me think more about integrating these features with other trace
backends. See below for short-term impact.

> Christophe: how does this differ from regular trace events and what
> extra information is needed?

- Grouping, as indicated above, mostly useful in practice to make selection
  of tracing topics easy (e.g. "modules") but also for real-time graphing,
  because typically a state change occurs in different functions, which is
  why I used locking as an example.

- Self-documentation. Right now, the recorder back-end generates rather
  unhelpful help messages.

- Trace buffer size. This is important to make post-mortem dumps usable if
  you record infrequent events alongside much higher-rate ones. For example,
  you may want a large buffer to record info about command-line option
  processing, the default 8 is definitely too small.

- Support for %+s, which tells that a string is safe to print later (e.g. it
  is a compile-time constant, or never ever freed).

- Support for custom formats, e.g. I use %v in the XL compiler for LLVM
  value pointers. This is a bit more advanced, but it would be neat to be
  able to print out QOM objects using %q :-)

For the short term, what about providing trace-named wrappers around these
additional recorder features?

--
Cheers,
Christophe de Dinechin (IRC c3d)