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Subject[PATCH v2]: perf/x86: expose user space frame-pointer value on a sample
Date

Store user space frame-pointer value (BP register) into Perf trace
on a sample for a process so the value becomes available when
unwinding call stacks for functions gaining event samples.

Test executable for the example below was compiled with frame pointer
support enabled:

g++ -o futex-fp -fpermissive --no-omit-frame-pointer futex.c

and profiled using:

tools/perf/perf record --user-regs=IP,SP,BP \
-g --call-graph=dwarf,1024 -e cycles -- ./futex-fp

Output of

tools/perf/perf report -i perf.data --stdio

demonstrates the effect of the patch change so before saving BP
value on a sample we have several frames missing above main
function frame:

# Samples: 138K of event 'cpu-cycles'
# Event count (approx.): 92713835335
#
# Children Self Command Shared Object Symbol
# ........ ........ ........ ................ ..........................
#
96.15% 0.72% futex-fp futex-fp [.] main
|
|--95.43%--main
| |
| |--71.56%--syscall
| | |
| | |--57.28%--entry_SYSCALL_64_after_hwframe
| | | |
| | | --56.95%--do_syscall_64
| | | |
| | | --55.77%--sys_futex

and after saving BP value on a sample we have expected

_start
__libc_start_main

frames unwound:

# Samples: 128K of event 'cpu-cycles'
# Event count (approx.): 85349981034
#
# Children Self Command Shared Object Symbol
# ........ ........ ........ ................ ..................
#
95.83% 0.00% futex-fp futex-fp [.] _start
|
==> ---_start
==> __libc_start_main
main
|
|--71.28%--syscall
| |
| |--55.67%--entry_SYSCALL_64
| | |
| | --55.40%--do_syscall_64
| | |
| | --54.21%--sys_futex


Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
---
Changes in v2:
- lifted restriction on frame pointer architecture so it's value is provided
as for i386 as for x86_64 processes

MAINTAINERS file lacks references to appropriate folks for reviewing
changes at arch/x86/kernel/perf_regs.c so probably it makes sense to
update the file as well in this respect.
---
arch/x86/kernel/perf_regs.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/perf_regs.c b/arch/x86/kernel/perf_regs.c
index e47b2dbbdef3..8d68658eff7f 100644
--- a/arch/x86/kernel/perf_regs.c
+++ b/arch/x86/kernel/perf_regs.c
@@ -156,7 +156,13 @@ void perf_get_regs_user(struct perf_regs *regs_user,
* Most system calls don't save these registers, don't report them.
*/
regs_user_copy->bx = -1;
- regs_user_copy->bp = -1;
+ /*
+ * Store user space frame-pointer value on sample
+ * to facilitate stack unwinding for cases when
+ * user space executable code has such support
+ * enabled at compile time;
+ */
+ regs_user_copy->bp = user_regs->bp;
regs_user_copy->r12 = -1;
regs_user_copy->r13 = -1;
regs_user_copy->r14 = -1;
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