[lttng-dev] [PATCH lttng-modules] Update kernel probes to more detailed match to kernel versions

[Mathieu Desnoyers]

* Gabbasov, Andrew (Andrew_Gabbasov at mentor.com) wrote:
> Hi Mathieu,
> 
> > * Andrew Gabbasov (andrew_gabbasov at mentor.com) wrote:
> > > We have added some more ifdef's to kernel probes to more closely match
> > > the kernel tracepoints for different kernel versions. The changes cover
> > > kernel versions from 2.6.38 up to 3.7. We did not track the earlier
> > > versions, and they are filtered out in probes Makefile.
> > >
> > > ext3 probe was modified to use #include <../fs/ext3/ext3.h> instead of
> > > copying this include from kernel source to lttng-modules source.
> > > Using such includes (there will be more similar in other probes)
> > > imposes a restriction to use the full kernel source for building
> > > lttng-modules rather than having just kernel "include" directory,
> > > but it looks like full source is required anyway, at least with respect
> > > to Makefiles structure. May be it is worth mentioning somewhere
> > > in the documentation.
> > >
> > > The code was verified to compile with latest versions in all stable
> > > branches from 2.6.38.x to 3.6.x and 3.7-rc7.
> > 
> > I get this build failure on 3.5 with this patch:
> > 
> >   CC [M]  /home/compudj/work/lttng-modules/probes/lttng-probe-ext3.o
> > /home/compudj/work/lttng-modules/probes/lttng-probe-ext3.c:30:29: fatal
> > error: ../fs/ext3/ext3.h: No such file or directory
> > compilation terminated.
> > 
> > can you look into it and submit a revised patch ?
> 
> You must be using kernel headers for building lttng-modules
> (linux-headers package or something like that), right? This is what I
> was writing about: since ext3.h is not available in kernel's "include"
> directory, and in order to avoid duplicating this file in
> lttng-modules sources, we have to use that <../> based reference, and
> this requires having full kernel source for building. The same issue
> will later be with btrfs and ext4 probes: their necessary include
> files are also not available in kernel headers. 
> 
> Trying to duplicate these header files to lttng-modules source seems
> to be not a good idea, especially if we need to add many ifdef's to
> them to track different kernel versions. We can try to insert some
> checks to the Makefile and skip building those probes if we do not
> have full kernel sources (actually if we do not have necessary include
> files). Or to declare having full kernel source as a requirement for
> building lttng-modules. What do you think?

The user experience should be that if lttng-modules is built against
linux-headers packages, the build should work fine, even if it means
some extra features are disabled.

And we don't want local copies of kernel headers, because then we could
trigger runtime bugs if the headers don't match between the kernel
version and local version.

> 
> > Also, although the README file states that we support only 3.6.38+, we
> > happen to also have support for kernels down to 2.6.32 with the patches
> > in the linux-patches/ directory applied to the appropriate kernel
> > versions. So it would be good that the instrumentation compiles for
> > those older kernels too. I promise we won't go further back than 2.6.32
> > though.
> 
> We'll try to add the ifdef's for earlier version down to 2.6.32 to the
> probes, although the additional amount of conditions makes the code
> overcomplicated ;-)

Indeed, but I never said supporting various kernels was easy ;)

I don't care immensely if older kernels have less features: in some
cases, it can be just OK to disable an entire tracepoint probe for older
kernels. I just want the build to work fine for those.

By the way, we're planning to go for a 2.1 release in a matter of days.
At that point, I'll have to decide if we keep the recent instrumentation
commits introduced after the start of the -rc cycle in this release (if
mature enough), or if we should branch before those for the 2.1 release
if they need more testing (and keep these instrumentation extensions for
2.2). Any thoughts on this ?

Thanks,

Mathieu

> 
> > Thanks,
> > 
> > Mathieu
> 
> Thanks.
> 
> Best regards,
> Andrew Gabbasov
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com