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

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Date:	Wed, 18 Jun 2014 13:53:55 +0100
From:	Will Deacon <will.deacon@....com>
To:	Jean Pihet <jean.pihet@...aro.org>
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 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?

> 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?

Will
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