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]".
Before:
$ 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 | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)
diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index b2ec38bf211e..d9f828f3b54f 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -1060,10 +1060,14 @@ static int machine__process_kernel_mmap_event(struct machine *machine,
strlen(kmmap_prefix));
/*
* Should be there already, from the build-id table in
- * the header.
+ * the header (but maybe with a different name: "vmlinux").
*/
- struct dso *kernel = __dsos__findnew(&machine->kernel_dsos,
- kmmap_prefix);
+ struct dso *kernel = dsos__find(&machine->kernel_dsos,
+ "vmlinux", true);
+
+ if (kernel == NULL)
+ kernel = __dsos__findnew(&machine->kernel_dsos,
+ kmmap_prefix);
if (kernel == NULL)
goto out_problem;
--
2.1.0
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