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SubjectRe: Re: [PATCH v2 00/16] perf: add memory access sampling support
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On Wed, Nov 7, 2012 at 3:53 PM, Masami Hiramatsu
<masami.hiramatsu.pt@hitachi.com> wrote:
> (2012/11/07 5:52), Arnaldo Carvalho de Melo wrote:
>> Em Mon, Nov 05, 2012 at 02:50:47PM +0100, Stephane Eranian escreveu:
>>> Or if one is interested in the data view:
>>> $ perf mem -t load rep --sort=symbol_daddr,cost
>>> # Samples: 19K of event 'cpu/mem-loads/pp'
>>> # Total cost : 1013994
>>> # Sort order : symbol_daddr,cost
>>> #
>>> # Overhead Samples Data Symbol Cost
>>> # ........ ........... ...................... .......
>>> #
>>> 0.10% 1 [.] 0x00007f67dffe8038 986
>>> 0.09% 1 [.] 0x00007f67df91a750 890
>>> 0.08% 1 [.] 0x00007f67e288fba8 826
>>>
>>
>>> CAVEAT: Note that the data addresses are not resolved correctly currently due to a
>>> problem in perf data symbol resolution code which I have not been able to
>>> uncover so far.
>>
>> Stephane,
>>
>> Those data addresses mostly are on the stack, we need reverse
>> resolution using DWARF location expressions to figure out what is the
>> name of a variable that is on a particular address, etc.
>>
>> Masami, have you played with this already? I mean:
>
> No, but it looks interesting. I'll try :)
>
>>
>> [root@sandy acme]# perf mem -t load rep --stdio --sort=symbol,symbol_daddr,cost
>> # Samples: 30 of event 'cpu/mem-loads/pp'
>> # Total cost : 640
>> # Sort order : symbol,symbol_daddr,cost
>> #
>> # Overhead Samples Symbol Data Symbol Cost
>> # ........ ........... ...................... ...................... .......
>> #
>> 55.00% 1 [k] lookup_fast [k] 0xffff8803b7521bd4 352
>> 5.47% 1 [k] cache_alloc_refill [k] 0xffff880407705024 35
>> 3.44% 1 [k] cache_alloc_refill [k] 0xffff88041d8527d8 22
>> 3.28% 1 [k] run_timer_softirq [k] 0xffff88041e2c3e90 21
>> 2.50% 1 [k] __list_add [k] 0xffff8803b7521d68 16
>> 2.19% 1 [.] __strcoll_l [.] 0x00007fffa8d44080 14
>> 1.88% 1 [.] __strcoll_l [.] 0x00007fffa8d44104 12
>>
>> If we go to the annotation browser to see where is that lookup_fast hitting we get:
>>
>> 100.00 │ mov -0x34(%rbp),%eax
>>
>> How to map 0xffff8803b7521bd4 to a stack variable, struct members and all?
>
> If perf stores %rbp value, we can do forward searching from local variables
> in this scope block. In some cases, the memory is dereferenced by another
> pointer. In that case, it is hard to do that (perhaps, we need to disassemble
> it and reverse execution). But in most case of the memory address on a stack,
> it will work, I think.
>
PEBS-LL does store rbp but it is not yet exposed. That will be in a follow-up
patch.

> Thank you,
>
>>
>> Humm, for userspace we have PERF_SAMPLE_REGS_USER for the dwarf unwinder we
>> need for userspace, but what about reverse mapping of kernel variables? Jiri?
>>
>> - Arnaldo
>> --
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>>
>
>
> --
> Masami HIRAMATSU
> IT Management Research Dept. Linux Technology Center
> Hitachi, Ltd., Yokohama Research Laboratory
> E-mail: masami.hiramatsu.pt@hitachi.com
>
>
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