Re: [Qemu-devel] [PATCH for-2.8 v1 60/60] trace: update docs to reflect new code generation approach

[Daniel P. Berrange]

On Wed, Aug 10, 2016 at 06:11:24PM +0200, Lluís Vilanova wrote:
> Daniel P Berrange writes:
> 
> > On Wed, Aug 10, 2016 at 04:33:47PM +0200, Lluís Vilanova wrote:
> >> Daniel P Berrange writes:
> >> 
> >> > Describe use of per-subdir trace events files and how it impacts
> >> > code generation.
> >> 
> >> > Signed-off-by: Daniel P. Berrange <address@hidden>
> >> > ---
> >> >  docs/tracing.txt | 61 
> >> > +++++++++++++++++++++++++++++++++++++++++++++++---------
> >> >  1 file changed, 52 insertions(+), 9 deletions(-)
> >> 
> >> > diff --git a/docs/tracing.txt b/docs/tracing.txt
> >> > index 29f2f9a..b7ba179 100644
> >> > --- a/docs/tracing.txt
> >> > +++ b/docs/tracing.txt
> >> > @@ -27,18 +27,51 @@ for debugging, profiling, and observing execution.
> >> 
> >> >  == Trace events ==
> >> 
> >> > +=== Sub-directory setup ===
> >> > +
> >> >  Each directory in the source tree can declare a set of static trace 
> >> > events
> >> > -in a "trace-events" file. Each trace event declaration names the event, 
> >> > its
> >> > -arguments, and the format string which can be used for pretty-printing:
> >> > +in a "trace-events" file. The first (non-comment) statement in the file
> >> > +must be "@id_offset(NN)" where NN is an integer that is unique among all
> >> > +"trace-events" files in the QEMU source tree. The IDs can be arbitrarily
> >> > +chosen, but extra fun the current "trace-events" files use values from 
> >> > the
> >> > +"powerful number" integer sequence :-)
> >> 
> >> Can we get rid of the "@id_offset(NN)"? If we're generating a 
> >> trace-events-all
> >> all the same, we can use that to generate all the per-directory file *and* 
> >> still
> >> keep the simpler flat event ids.
> 
> > One thing that occurred to me is that we could take a hash of the
> > group name. Since we have few enough sub-dirs / groups, and a large
> > enough ID space, it ought to be possible to get a hash which is
> > unique. The trace tool can double check that all groups get a unique
> > hash
> 
> Eliminating "@id_offset" is nice, but I would prefer to eliminate the event
> hierarchy too.

Having the event groups is the only way to avoid re-numbering all events
when a new one is added.

> >> For example, first we concatenate /trace-events, /io/trace-events and
> >> /hw/usb/trace-events into /trace-events-all to produce:
> >> 
> >> @group core
> >> @dir /
> >> # contents of /trace-events
> >> @group io
> >> @dir /io
> >> # contents of /io/trace-events
> >> @group hw_usb
> >> @dir /hw/usb
> >> # contents of /hw/usb/trace-events
> >> 
> >> Then we can use /trace-events-all to generate all the per-directory trace.h
> >> files while ensuring a perfectly flat event identifier space (a single
> >> generated-events.c is used).
> >> 
> >> This should minimize changes to tracing code and avoid further restrictions
> >> (like the "top-level vcpu" one).
> 
> > That means when any individual trace file is changed, we re-generate the
> > world, which is exactly what this series is designed to avoid.
> 
> Nope. We re-generate /trace-events-all, but each individual file generated
> afterwards (taken from the "@dir" directive) will change only if its new
> contents do (the Makefile uses cmp to check if files need updating from their
> corresponding timestamp file).

The contents will still need to change too if we have a global numbering
space as your suggestion requires. The event groups are the only way to
avoid that.

Regards,
Daniel
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