Re: Perf ABI versioning

Re: Perf ABI versioning

[Frederic Weisbecker]

(Adding LKML and more tracing people in Cc)

On Mon, Jan 24, 2011 at 05:04:00PM +0100, Thomas Renninger wrote:
> Hi,
> 
> as I recently cleaned up the power events with
> some format/abi adjustings...
> 
> I realized that it was rather easy to adjust
> the stuff in drivers/tools/perf, because there
> are some nice facilities to query which events
> are available.
> 
> I looked at pytimechart, there it's not that nice
> and convenient would be some kind of version number.
> 
> pytimechart can parse traces which are simply copied
> away from /sys/kernel/debug/trace and to check for
> available events and their format, one would have to
> do a separate, ugly to implement, iteration of parsing.
> 
> Would it make sense to introduce a trace
> ABI version number?
> It would be enough to increase it by one every time
> any event (don't know about other perf facilities) is
> added, removed or modified.
> Ideally the modifications are tracked with a short
> description in a file like Documentation/trace/format.changes
> 
> Untested, but the patch at the end should simply add
> a version number on top of
> /sys/kernel/debug/trace file
> 
> While this should be enough for the kernel, perf record
> would need to include it into it's binary perf.data format
> as well?
> 
> Then userspace apps could easily test whether supported
> events would be included at all, whether only a subset of
> supported events were made available and which format they have.
> 
> Does this make sense?

This may be generally useful to help dealing with tracepoint ABI
changes.

But instead of a global tracing ABI number, I would rather suggest
one number per tracepoint subsystem (sched, power, etc...).

Ideally it would be per event, but sometimes those events tend to be
renamed or a whole tracepoint subsystem refactored (see workqueue
lately). Hence it might be better per subsystem.

Now these numbers can be increased in the case of new fields added,
fields removed, fields renamed, new tracepoints, removed tracepoints,
renamed tracepoints.

But I think we shouldn't do that for pure ascii format changes, because
we created the format to avoid fixing that on the rock. I know that
would help Pytimechart, and it's a precious tool that I wish we can help
the development. But we shouldn't encourage the pattern match way of
parsing events, instead of using the format file.

Also, tracking the changes in the Documentation is going to be a
nightmare. I suggest developers who want to dig into details of
an ABI change to check the code or the format themselves.

And BTW we can probably provide more granularity in the versioning
with a major number for ABI changes that may break tools (field or
tracepoint removal, rename, etc...) and a minor number for changes
that probably won't break tools but could bring them more informations:
new tracepoint fields, new tracepoints, etc... so we can tell the tools
about that.

Hm?

Re: Perf ABI versioning

[Peter Zijlstra]

On Mon, 2011-01-24 at 21:34 +0100, Frederic Weisbecker wrote:
> 
> But instead of a global tracing ABI number, I would rather suggest
> one number per tracepoint subsystem (sched, power, etc...).
> 
> Ideally it would be per event, but sometimes those events tend to be
> renamed or a whole tracepoint subsystem refactored (see workqueue
> lately). Hence it might be better per subsystem. 

What's wrong with what we have? the /format file is pretty unique to
function as a version number of you use a hash over it.

Re: Perf ABI versioning

[Frederic Weisbecker]

On Mon, Jan 24, 2011 at 09:39:03PM +0100, Peter Zijlstra wrote:
> On Mon, 2011-01-24 at 21:34 +0100, Frederic Weisbecker wrote:
> > 
> > But instead of a global tracing ABI number, I would rather suggest
> > one number per tracepoint subsystem (sched, power, etc...).
> > 
> > Ideally it would be per event, but sometimes those events tend to be
> > renamed or a whole tracepoint subsystem refactored (see workqueue
> > lately). Hence it might be better per subsystem. 
> 
> What's wrong with what we have? the /format file is pretty unique to
> function as a version number of you use a hash over it.

Yeah we could in fact use it to find if fields have been added,
removed. Right, all in one that looks enough to me.

Re: Perf ABI versioning

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> This may be generally useful to help dealing with tracepoint ABI changes.
> 
> But instead of a global tracing ABI number, I would rather suggest one number per 
> tracepoint subsystem (sched, power, etc...).

Nooooooooooo ... !!! :-)

Please lets stop this madness before it gets too serious: we dont do ABI version 
numbering in Linux, full stop.

We use 'natural' ABIs where the lack of an ABI component triggers some sort of 
clean, finegrained error. Like a -EINVAL on a not-yet-implemented ABI component, a 
non-existent file entry, or -ENOSYS on a non-existent syscall.

Such a design is arbitrarily backportable or forward portable, it's extensible and 
it is actually maintainable.

In the ABI version numbering direction lies Windows madness ...

Thanks,

	Ingo

Re: Perf ABI versioning

[Thomas Renninger]

On Monday 24 January 2011 22:28:38 Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > This may be generally useful to help dealing with tracepoint ABI changes.
> > 
> > But instead of a global tracing ABI number, I would rather suggest one number per 
> > tracepoint subsystem (sched, power, etc...).
> 
> Nooooooooooo ... !!! :-)
> 
> Please lets stop this madness before it gets too serious: we dont do ABI version 
> numbering in Linux, full stop.
Ok... :)
 
> We use 'natural' ABIs where the lack of an ABI component triggers some sort of 
> clean, finegrained error. Like a -EINVAL on a not-yet-implemented ABI component, a 
> non-existent file entry, or -ENOSYS on a non-existent syscall.
If the whole stuff matures, someone might think about a nice concept to be able
to shift around perf.data binaries which can be processed by userspace tools,
independent of the kernel version running below on which the perf.data
was produced.

Was just an idea, I agree that it's far away from being practical,
please ignore the thread...

Sorry for the noise,

   Thomas

Re: Perf ABI versioning

[Arnaldo Carvalho de Melo]

Em Mon, Jan 24, 2011 at 10:49:13PM +0100, Thomas Renninger escreveu:
> On Monday 24 January 2011 22:28:38 Ingo Molnar wrote:
> > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > 
> > > This may be generally useful to help dealing with tracepoint ABI changes.
> > > 
> > > But instead of a global tracing ABI number, I would rather suggest one number per 
> > > tracepoint subsystem (sched, power, etc...).
> > 
> > Nooooooooooo ... !!! :-)
> > 
> > Please lets stop this madness before it gets too serious: we dont do ABI version 
> > numbering in Linux, full stop.
> Ok... :)
>  
> > We use 'natural' ABIs where the lack of an ABI component triggers some sort of 
> > clean, finegrained error. Like a -EINVAL on a not-yet-implemented ABI component, a 
> > non-existent file entry, or -ENOSYS on a non-existent syscall.
> If the whole stuff matures, someone might think about a nice concept to be able
> to shift around perf.data binaries which can be processed by userspace tools,
> independent of the kernel version running below on which the perf.data
> was produced.

We try to do that already, things like sample_id_all that is a feature
present on a new kernel are capability queried by trying to ask the
kernel for it and if it fails, cope with that and do as before it, in
some cases warning the user if the fallback can lead to inacuracies.

Shifting around perf.data files accross arquitectures and OS/vendor
releases is an everyday goal.

I.e. the new perf binary works with older kernels.

We also try hard to keep the latest tools building on older kernels.

Using the /format file hashed as peterz said is elegant, fine grained
content based ABI number :-)

- Arnaldo

Re: Perf ABI versioning

[Thomas Renninger]

On Monday 24 January 2011 22:57:55 Arnaldo Carvalho de Melo wrote:
> Em Mon, Jan 24, 2011 at 10:49:13PM +0100, Thomas Renninger escreveu:
... 
> Using the /format file hashed as peterz said is elegant, fine grained
> content based ABI number :-)
Ok thanks, I should have had a closer look at the binary format.

It may make sense at some time to provide a library so that other tools
can make use of the most convenient functions in tools/perf/utils/*
as well?

I'll have a look at the powertop perf implementations I wanted to
look at anyway and try to understand the stuff a bit better.

Thanks,

       Thomas

Re: Perf ABI versioning

[Frederic Weisbecker]

On Mon, Jan 24, 2011 at 11:30:01PM +0100, Thomas Renninger wrote:
> On Monday 24 January 2011 22:57:55 Arnaldo Carvalho de Melo wrote:
> > Em Mon, Jan 24, 2011 at 10:49:13PM +0100, Thomas Renninger escreveu:
> ... 
> > Using the /format file hashed as peterz said is elegant, fine grained
> > content based ABI number :-)
> Ok thanks, I should have had a closer look at the binary format.
> 
> It may make sense at some time to provide a library so that other tools
> can make use of the most convenient functions in tools/perf/utils/*
> as well?

I personally wish the other way around: having Pytimechart merged in perf :)
If I remember correctly, the problem was that we don't have support for
trace_printk with perf?

But sure if more out of tree tools show up and want to use tracepoints,
we can think about librarizing it.

(Even though my secret hope is that
we build a nice one tool that can answer most tracing needs together rather
than many scattered ad hoc pieces.)

Re: Perf ABI versioning

[Arnaldo Carvalho de Melo]

Em Mon, Jan 24, 2011 at 11:46:12PM +0100, Frederic Weisbecker escreveu:
> On Mon, Jan 24, 2011 at 11:30:01PM +0100, Thomas Renninger wrote:
> > On Monday 24 January 2011 22:57:55 Arnaldo Carvalho de Melo wrote:
> > > Em Mon, Jan 24, 2011 at 10:49:13PM +0100, Thomas Renninger escreveu:
> > ... 
> > > Using the /format file hashed as peterz said is elegant, fine grained
> > > content based ABI number :-)
> > Ok thanks, I should have had a closer look at the binary format.
> > 
> > It may make sense at some time to provide a library so that other tools
> > can make use of the most convenient functions in tools/perf/utils/*
> > as well?
> 
> I personally wish the other way around: having Pytimechart merged in perf :)
> If I remember correctly, the problem was that we don't have support for
> trace_printk with perf?
> 
> But sure if more out of tree tools show up and want to use tracepoints,
> we can think about librarizing it.
> 
> (Even though my secret hope is that
> we build a nice one tool that can answer most tracing needs together rather
> than many scattered ad hoc pieces.)

Yes, there is this conflict of tools/perf/ people coming from the
kernel, where we can change APIs as we see fit as long as we pay the
price of going over the users to fix them up.

And the desire of having something that can be used by projects living
outside the kernel source repo.

There are at least two efforts underway that are paving the way to
having something usable by third parties, Borislav Petkov's patchseries
to have some liblk/libtrace and my work for .39 to have a python perf.so
binding.

I think we should try as hard as possible to get things in tools/ so
that we have as long an experience in trying to get good abstractions as
possible before we decide on a v1 "outsiders API" :-)

- Arnaldo

P.S. Most of my work is in my perf/core branch in my git.k.o tree,
both mine and Borislavs are a wersearch away :)

Re: Perf ABI versioning

[Frederic Weisbecker]

On Mon, Jan 24, 2011 at 10:28:38PM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > This may be generally useful to help dealing with tracepoint ABI changes.
> > 
> > But instead of a global tracing ABI number, I would rather suggest one number per 
> > tracepoint subsystem (sched, power, etc...).
> 
> Nooooooooooo ... !!! :-)
> 
> Please lets stop this madness before it gets too serious: we dont do ABI version 
> numbering in Linux, full stop.
> 
> We use 'natural' ABIs where the lack of an ABI component triggers some sort of 
> clean, finegrained error. Like a -EINVAL on a not-yet-implemented ABI component, a 
> non-existent file entry, or -ENOSYS on a non-existent syscall.
> 
> Such a design is arbitrarily backportable or forward portable, it's extensible and 
> it is actually maintainable.
> 
> In the ABI version numbering direction lies Windows madness ...
> 
> Thanks,
> 
> 	Ingo

Yeah. But the tracepoint case looked to me quite special as sometimes
the ABI doesn't evolved smoothly like in the scheme you describe (see
the power events case).

OTOH, checking the format file if we have lost or gained fields must be enough
already to, indeed, avoid a versioning madness :)