Re: [PATCH v1] perf record: collect user registers set jointly with dwarf stacks

[Alexey Budankov]

Hi Arnaldo,

On 17.04.2019 18:48, Arnaldo Carvalho de Melo wrote:
> On April 17, 2019 11:40:02 AM GMT-03:00, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>> On Wed, Apr 17, 2019 at 11:35:42AM -0300, Arnaldo Carvalho de Melo
>> wrote:
>>> Em Wed, Apr 17, 2019 at 09:39:52AM +0200, Jiri Olsa escreveu:
>>>> On Mon, Apr 15, 2019 at 06:36:13PM +0300, Alexey Budankov wrote:
>>>>>
>>>>> When dwarf stacks are collected jointly with user specified
>> register
>>>>> set using --user-regs option like below the full register context
>> is
>>>>> still captured on a sample:
>>>>>
>>>>>   $ perf record -g --call-graph dwarf,1024 --user-regs=IP,SP,BP
>> -- matrix.gcc.g.O3
>>>>>
>>>>>   188143843893585 0x6b48 [0x4f8]: PERF_RECORD_SAMPLE(IP, 0x4002):
>> 23828/23828: 0x401236 period: 1363819 addr: 0x7ffedbdd51ac
>>>>>   ... FP chain: nr:0
>>>>>   ... user regs: mask 0xff0fff ABI 64-bit
>>>>>   .... AX    0x53b
>>>>>   .... BX    0x7ffedbdd3cc0
>>>>>   .... CX    0xffffffff
>>>>>   .... DX    0x33d3a
>>>>>   .... SI    0x7f09b74c38d0
>>>>>   .... DI    0x0
>>>>>   .... BP    0x401260
>>>>>   .... SP    0x7ffedbdd3cc0
>>>>>   .... IP    0x401236
>>>>>   .... FLAGS 0x20a
>>>>>   .... CS    0x33
>>>>>   .... SS    0x2b
>>>>>   .... R8    0x7f09b74c3800
>>>>>   .... R9    0x7f09b74c2da0
>>>>>   .... R10   0xfffffffffffff3ce
>>>>>   .... R11   0x246
>>>>>   .... R12   0x401070
>>>>>   .... R13   0x7ffedbdd5db0
>>>>>   .... R14   0x0
>>>>>   .... R15   0x0
>>>>>   ... ustack: size 1024, offset 0xe0
>>>>>    . data_src: 0x5080021
>>>>>    ... thread: stack_test2.g.O:23828
>>>>>    ...... dso: /root/abudanko/stacks/stack_test2.g.O3
>>>>>
>>>>> After applying the change suggested in the patch the sample data
>> contain
>>>>> only user specified register values:
>>>>>
>>>>>   $ perf record -g --call-graph dwarf,1024 --user-regs=IP,SP,BP
>> -- matrix.gcc.g.03
>>>>>
>>>>>   188368474305373 0x5e40 [0x470]: PERF_RECORD_SAMPLE(IP, 0x4002):
>> 23839/23839: 0x401236 period: 1260507 addr: 0x7ffd3d85e96c
>>>>>   ... FP chain: nr:0
>>>>>   ... user regs: mask 0x1c0 ABI 64-bit
>>>>>   .... BP    0x401260
>>>>>   .... SP    0x7ffd3d85cc20
>>>>>   .... IP    0x401236
>>>>>   ... ustack: size 1024, offset 0x58
>>>>>    . data_src: 0x5080021
>>>>>    ... thread: stack_test2.g.O:23839
>>>>>    ...... dso: /root/abudanko/stacks/stack_test2.g.O3
>>>>>
>>>>> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
>>>>
>>>> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
>>>
>>> So, there are registers that are needed to do the DWARF unwinding,
>>> right? But at the same time, if the user says only some are needed,
>> he
>>> better know what they're doing and ask for at least the registers
>> needed
>>> for the unwinding process to be successfull, right?
>>
>> yep, that's how understand that
> 
> So we need to document that, stating that specifying a set of registers together with requesting DWARF callchains may break things.

Do you mean break callchains if omitting IP,SP,BP? 
For example like this: 
$ perf record -g --call-graph dwarf,1024 --user-regs=AX,BX,CX -- matrix.gcc.g.O3

~Alexey

> 
> - Arnaldo
> 
> 
> 
> 
>