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
(which might break the result). I think it's worth going to the
stable tree.
I can think of a couple of ways to fix it. In this patch, I changed
to use the name of "[kernel.kallsyms]" for the kernel build-id event
instead of not trying vmlinux paths. This way we can provide maximum
info (like annotation) with minimum change IMHO.
Before:
$ perf record -a usleep 1
$ perf buildid-list
00d5ff078efe1d30b8492854f259215fd877ce30 /lib/modules/3.16.0-rc2+/build/vmlinux
78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
$ perf buildid-list -H
0000000000000000000000000000000000000000 [kernel.kallsyms]
78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
0000000000000000000000000000000000000000 /tmp/perf-2523.map
After:
$ perf record -a usleep 1
$ perf buildid-list
00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]
78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
$ perf buildid-list -H
00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]
78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
0000000000000000000000000000000000000000 /tmp/perf-2523.map
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
tools/perf/util/header.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 158c787ce0c4..5c4093dee467 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -263,6 +263,9 @@ static int __dsos__write_buildid_table(struct list_head *head,
machine__mmap_name(machine, nm, sizeof(nm));
name = nm;
name_len = strlen(nm) + 1;
+ } else if (dso__is_vmlinux(pos)) {
+ name = pos->name;
+ name_len = strlen(pos->name) + 1;
} else {
name = pos->long_name;
name_len = pos->long_name_len + 1;
--
2.0.0
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