[PATCH v3] perf tools: Fix build-id matching on vmlinux

From: Namhyung Kim
Date: Sun Sep 21 2014 - 22:46:04 EST

There's a problem on finding correct kernel symbols when perf report
runs on a different kernel. Although a part of the problem was solved
by the prior commit 0a7e6d1b6844 ("perf tools: Check recorded kernel
version when finding vmlinux"), there's a remaining problem still.

When perf records samples, it synthesizes the kernel map using
machine__mmap_name() and ref_reloc_sym like "[kernel.kallsyms]_text".
You can easily see it using 'perf report -D' command.

After finishing record, it goes through the recorded events to find
maps/dsos actually used. And then record build-id info of them.

During this process, it needs to load symbols in a dso and it'd call
dso__load_vmlinux() since the default value of the symbol_conf.try_
vmlinux_path is true. However it changes dso->long_name to a real
path of the vmlinux file (e.g. /lib/modules/3.16.0-rc2+/build/vmlinux)
if one is running on a custom kernel.

It resulted in that perf report reads the build-id of the vmlinux, but
cannot use it since it only knows about the [kernel.kallsyms] map. It
then falls back to possible vmlinux paths by using the recorded kernel
version (in case of a recent version) or a running kernel silently.

Even with the recent tools, this still has a possibility of breaking
the result. As the build directory is a symbolic link, if one built a
new kernel in the same directory with different source/config, the old
link to vmlinux will point the new file. So it's absolutely needed to
use build-id when finding a kernel image.

In this patch, it's now changed to try to search a kernel dso using
"vmlinux" shortname (which always has a build-id) and, if not found,
search "[kernel.kallsyms]".


$ perf report
# Children Self Command Shared Object Symbol
# ........ ........ ....... ................. ...............................
72.15% 0.00% swapper [kernel.kallsyms] [k] set_curr_task_rt
72.15% 0.00% swapper [kernel.kallsyms] [k] native_calibrate_tsc
72.15% 0.00% swapper [kernel.kallsyms] [k] tsc_refine_calibration_work
71.87% 71.87% swapper [kernel.kallsyms] [k] module_finalize

After (for the same perf.data):

72.15% 0.00% swapper vmlinux [k] cpu_startup_entry
72.15% 0.00% swapper vmlinux [k] arch_cpu_idle
72.15% 0.00% swapper vmlinux [k] default_idle
71.87% 71.87% swapper vmlinux [k] native_safe_halt

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
tools/perf/util/machine.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index b2ec38bf211e..14880b81e98c 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -1062,8 +1062,22 @@ static int machine__process_kernel_mmap_event(struct machine *machine,
* Should be there already, from the build-id table in
* the header.
- struct dso *kernel = __dsos__findnew(&machine->kernel_dsos,
- kmmap_prefix);
+ struct dso *kernel = NULL;
+ struct dso *dso;
+ list_for_each_entry(dso, &machine->kernel_dsos, node) {
+ const char *suffix;
+ suffix = dso->long_name + strlen(dso->long_name) - 3;
+ if (strcmp(suffix, ".ko")) {
+ kernel = dso;
+ break;
+ }
+ }
+ if (kernel == NULL)
+ kernel = __dsos__findnew(&machine->kernel_dsos,
+ kmmap_prefix);
if (kernel == NULL)
goto out_problem;


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