linux-kernel - [PATCH 3/3] ARM: perf: allow tracing with kernel tracepoints events

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Date:	Mon,  7 Jul 2014 15:45:10 +0200
From:	Jean Pihet <jean.pihet@...aro.org>
To:	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org,
	Will Deacon <will.deacon@....com>
Cc:	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	steve.capper@...aro.org, Jean Pihet <jean.pihet@...aro.org>
Subject: [PATCH 3/3] ARM: perf: allow tracing with kernel tracepoints events

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:
 - program counter for PERF_SAMPLE_IP, as used by dwarf unwinding,
 - sp & fp for fp based callchain unwinding,
 - cpsr for user_mode() tests.

Tested with perf record and tracepoints triggering (-e <tracepoint>), with
unwinding using fp (--call-graph fp) and dwarf info (--call-graph dwarf).

Reported by Sneha Priya on linaro-dev, cf.
http://lists.linaro.org/pipermail/linaro-dev/2014-May/017151.html

Signed-off-by: Jean Pihet <jean.pihet@...aro.org>
Cc: Will Deacon <will.deacon@....com>
Cc: Steve Capper <steve.capper@...aro.org>
Reported-by: Sneha Priya <sneha.cse@...mail.com>
---
 arch/arm/include/asm/perf_event.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/arm/include/asm/perf_event.h b/arch/arm/include/asm/perf_event.h
index 7558775..b02b5d3 100644
--- a/arch/arm/include/asm/perf_event.h
+++ b/arch/arm/include/asm/perf_event.h
@@ -26,6 +26,25 @@ struct pt_regs;
 extern unsigned long perf_instruction_pointer(struct pt_regs *regs);
 extern unsigned long perf_misc_flags(struct pt_regs *regs);
 #define perf_misc_flags(regs)	perf_misc_flags(regs)
+
+/*
+ * Take a snapshot of the regs.
+ * We only need a few of the regs:
+ * - program counter for PERF_SAMPLE_IP, as used by dwarf unwinding,
+ * - sp & fp for fp based callchain unwinding,
+ * - cpsr for user_mode() tests.
+ */
+#define perf_arch_fetch_caller_regs(regs, __ip) {	\
+	instruction_pointer(regs)= (__ip);		\
+	__asm__ (					\
+		"str	sp, %[_ARM_sp]		\n\t"	\
+		"str	fp, %[_ARM_fp]		\n\t"	\
+		"mrs	%[_ARM_cpsr], cpsr	\n\t"	\
+		: [_ARM_sp]   "=m" (regs->ARM_sp),	\
+		  [_ARM_fp]   "=m" (regs->ARM_fp),	\
+		  [_ARM_cpsr] "=r" (regs->ARM_cpsr)	\
+	);						\
+}
 #endif
 
 #endif /* __ARM_PERF_EVENT_H__ */
-- 
1.8.1.2

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