Messages in this thread | | | Date | Mon, 30 Sep 2013 18:54:20 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] perf: mmap2 not covering VM_CLONE regions |
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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@infradead.org> 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.
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