linux-kernel - Re: [RFC] perf: mmap2 not covering VM

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Message-ID: <CABPqkBSRV651SynNHpAo7-B+8aX2hsMXKJNeTkimBoWo91TvFQ@mail.gmail.com>
Date:	Mon, 30 Sep 2013 18:48:55 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC] perf: mmap2 not covering VM_CLONE regions

On Mon, Sep 30, 2013 at 6:15 PM, Peter Zijlstra <peterz@...radead.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.
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