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

[Arnaldo Carvalho de Melo]

Em Sun, Sep 07, 2014 at 10:24:12PM +0200, Stephane Eranian escreveu:
> On Fri, Sep 5, 2014 at 5:44 PM, Arnaldo Carvalho de Melo
> <acme@xxxxxxxxxx> wrote:
> > Em Sat, Sep 06, 2014 at 12:16:40AM +0900, Namhyung Kim escreveu:
> >> Hi Adrian and Arnaldo,
> >>
> >> 2014-09-05 (ê), 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:
> >
> Can't you figure out it is from the kernel (in perf record) by remembering
> you got the buildid from /sys/kernel/notes?

Sure, the question was more about how to encode that in the build-id
table in the perf.data file, i.e. we don't have a flags field there that
we could use a bit for this purpose (hey, this entry is for the kernel,
please match it to the PERF_RECORD_MMAP [kernel_kallsyms] synthesized
for the kernel).

We know it is a kernel because the name is [kernel.kallsyms] in the
synthesized PERF_RECORD_MMAP. So perhaps we should have the kernel
pathname right after the reference relocation symbol, that way we would
use it to get the entry in the build-id table.

I.e. parts of what we need are in the synthesized PERF_RECORD_MMAP (how
to do relocation, start-end kernel mmap, and we have parts of it in the
build-id table: the build-id and the full pathname. How to connect both
is what we're trying to achieve here, and in a backwards compatible
way.

The (ugly) way I found to achieve all this is to concatenate the full
pathname right after the ref reloc symbol in the synthesized
PERF_RECORD_MMAP for the kernel mmap.

Older tools would just ignore what is after the ref reloc symbol (need
to check if we put the zero there, if not, we can add the zero + the
full pathname.).

Newer tools would look if the size of the record is longer than the
strlen(ref_reloc_symbol), and if it is, it means we have the kernel full
pathname after the ref_reloc_symbol, that we can use to find the
build-id in the build-id table.

In the future this is all moot if we add the build-id to all
PERF_RECORD_MMAP events, as described below:

> > 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
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