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

[Namhyung Kim]

Hi Arnaldo,

On Fri, 5 Sep 2014 12:44:02 -0300, Arnaldo Carvalho de Melo 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]$

Hmm.. this is a report option to specify a vmlinux file rather than
finding it using recorded info.  When this option was given by user, it
doesn't even try to search the vmlinux path (or build-id table) and just
uses the given path.


>
> 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.

Maybe we can add a new build_id2_event (like mmap2) that has a new field
to record that info.


> 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.

But it'd require a realtime processing of events at record time.

Thanks,
Namhyung
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