Re: [RFC] perf: mmap2 not covering VM_CLONE regions

[Stephane Eranian]

On Mon, Sep 30, 2013 at 6:54 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 30, 2013 at 06:48:55PM +0200, Stephane Eranian wrote:
> > On Mon, Sep 30, 2013 at 6:15 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > On Mon, Sep 30, 2013 at 05:44:41PM +0200, Stephane Eranian wrote:
> > >> Hi,
> > >>
> > >> I was alerted by people trying to use the PERF_RECORD_MMAP2
> > >> record to disambiguate virtual address mappings that there is a case
> > >> where the record does not contain enough information.
> > >>
> > >> As you know, the MMAP2 record adds the major, minor, ino number,
> > >> inode generation numbers to a mapping. But it does that only for
> > >> file or pseudo -file backed mappings. That covers file mmaps and also
> > >> SYSV shared memory segments.
> > >>
> > >> However there is a another kind of situation that arises in some
> > >> multi-process benchmarks where a region of memory is cloned
> > >> using VM_CLONE. As such, the virtual addresses match between
> > >> the processes but the major, minor, inode, inode generation  fields
> > >> are all zeroes because there is no inode associated with the mapping.
> > >> Yet, it is important for the tool to know the mappings between the
> > >> processes are pointing to the same physical data.
> > >>
> > >> We need to cover this case and I am seeking for advice on how to
> > >> best address this need given that we discarded using the plain physical
> > >> address for disambiguation.
> > >
> > > Urgh.. who in his bloody mind is playing VM_CLNOE games that is not
> > > pthread_creatE() ?
> >
> > Some matrix multiply benchmark, I guess.
>
> So the problem is that we don't have a user visible address space
> identifier; with CLONE_THREAD we have the thread group id that acts
> like this. But for bare CLONE_VM usage there's nothing afaik.


>From the tool's perspective, the MMAP2 record must contain enough information
to identify that the mapping points to the same physical pages in that
particular
case (multi-process + VM_CLONE). As we have it now all inode-related fields
are zero which is useless (indicates: no info). In other words, we need to make
up some unique number and stash it in the maj.min,ino triplet somehow.
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