Re: [PATCH v2] perf tools: Fix build-id matching on vmlinux

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Sat, Sep 06, 2014 at 12:16:40AM +0900, Namhyung Kim escreveu:
> Hi Adrian and Arnaldo,
> 
> 2014-09-05 (금), 11:11 -0300, Arnaldo Carvalho de Melo:
> > Em Fri, Sep 05, 2014 at 10:22:40AM +0300, Adrian Hunter escreveu:
> > > On 09/05/2014 07:59 AM, Namhyung Kim wrote:
> > > > +++ b/tools/perf/util/machine.c
> > > > @@ -1060,10 +1060,14 @@ static int machine__process_kernel_mmap_event(struct machine *machine,
> > > >  				strlen(kmmap_prefix));
> > > >  		/*
> > > >  		 * Should be there already, from the build-id table in
> > > > -		 * the header.
> > > > +		 * the header (but maybe with a different name: "vmlinux").
> > > >  		 */
> > > > -		struct dso *kernel = __dsos__findnew(&machine->kernel_dsos,
> > > > -						     kmmap_prefix);
> > > > +		struct dso *kernel = dsos__find(&machine->kernel_dsos,
> > > > +						"vmlinux", true);
> >  
> > > Isn't "vmlinux" just the basename of the original file name, so if it had a
> > > different name this wouldn't work e.g. if the filename had been
> > > /boot/vmlinuz-3.11.0-26-generic then you would need
> > > "vmlinuz-3.11.0-26-generic" for this to work?
> > 
> > Yeah, looking for well known pathnames to then check if the build-id
> > matches the one we're looking for, be it because we obtained it from
> > /sys/kernel/notes (for the running kernel), or from the perf.data file
> > build-id table is ok, as we don't know where it is.
> > 
> > Plain sticking "vmlinux" there is not.
> 
> I don't get it.  AFAIK when perf record runs, it uses [kernel.kallsyms]
> name for kernel map.  But it can be changed only if it found a "vmlinux"
> file in the vmlinux_path[].  So short name will always be "vmlinux" -
> okay, it might be vmlinux-$(uname -r) too; I can add it.
> 
> As you know, the vmlinux file is a ELF image that created during kernel
> build process so I guess its name is fixed.  It's different from the
> vmlinu"z" which resides in /boot directory.
> 
> Am I missing something?

[acme@zoo linux]$ perf report --help 2>&1 | grep vmlin
       -k, --vmlinux=<file>
           vmlinux pathname
[acme@zoo linux]$

Yes, it is using [kernel.kallsyms].ref_reloc_sym for the synthesized
KERNEL mmap event, but it stores the full path in the build-id table.

It seems we need a way to state that an entry in the build-id table is
for the kernel, without looking at its file name. That or to put the
build-id into the synthesized kernel mmap event. Which is better,
because:

That leads to another problem that needs to get solved eventually: We
need to have the build-id into PERF_RECORD_MMAP, because we're now using
just the mmap filename as the key, not the contents, and for long
running sessions, DSOs can get updated, etc.

- Arnaldo