linux-kernel - Re: [PATCH] ARM: perf: allow tracing with kernel tracepoints events

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Date:	Fri, 20 Jun 2014 10:10:35 +0200
From:	Jean Pihet <jean.pihet@...aro.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Sneha Priya <sneha.cse@...mail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: perf: allow tracing with kernel tracepoints events

Hi Will,

On 18 June 2014 14:53, Will Deacon <will.deacon@....com> wrote:
> Hi Jean,
>
> On Tue, Jun 17, 2014 at 06:11:05PM +0100, Jean Pihet wrote:
>> When tracing with tracepoints events the IP and CPSR are set to 0,
>> preventing the perf code to resolve the symbols:
>>
>> ./perf record -e kmem:kmalloc cal
>> [ perf record: Woken up 1 times to write data ]
>> [ perf record: Captured and wrote 0.007 MB perf.data (~321 samples) ]
>>
>> ./perf report
>> Overhead Command Shared Object Symbol
>> ........ ....... ............. ...........
>> 40.78%   cal     [unknown]     [.]00000000
>> 31.6%    cal     [unknown]     [.]00000000
>>
>> The examination of the gathered samples (perf report -D) shows the IP
>> is set to 0 and that the samples are considered as user space samples,
>> while the IP should be set from the registers and the samples should be
>> considered as kernel samples.
>>
>> The fix is to implement perf_arch_fetch_caller_regs for ARM, which
>> fills the necessary registers used for the callchain unwinding and
>> to determine the user/kernel space property of the samples: ip, sp, fp
>> and cpsr.
>
> Surely its only the CPSR that identifies whether it's user or kernel?
Yes, user_mode() is used to determine the user/kernel property of the
samples. user_mode is defined as (((regs)->ARM_cpsr & 0xf) == 0) in
ptrace.h.

>
>> Tested with perf record and tracepoints filtering (-e <tracepoint>), with
>> unwinding using fp (--call-graph fp) and dwarf info (--call-graph dwarf).
>
> Whilst the old ACPS unwinding only needs PC, FP and SP, is this definitely
> true for exidx and DWARF-based unwinding? Given that libunwind ends up
> running a state machine for the latter, can we guarantee that we won't hit
> instructions that require access to other general purpose registers?
Yes. dwarf unwinding does not need anything extra. Once seeded all the
rest is extracted from the dwarf trace info.

I am currently stress testing the change, let me come back to you with
the results.

Thx,
Jean

>
> Will
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