by tip-bot for Vasyl Gomonovych

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Subject: [tip:perf/core] perf probe: Try to use symbol table if searching debug info failed

Commit-ID: 1c0bd0e891aaed0219010bfe79b32e1b0b82d662
Gitweb: http://git.kernel.org/tip/1c0bd0e891aaed0219010bfe79b32e1b0b82d662
Author: Wang Nan <[email protected]>
AuthorDate: Fri, 21 Aug 2015 10:09:02 +0000
Committer: Arnaldo Carvalho de Melo <[email protected]>
CommitDate: Fri, 21 Aug 2015 12:57:20 -0300

perf probe: Try to use symbol table if searching debug info failed

A problem can occur in a statically linked perf when vmlinux can be found:

# perf probe --add sys_epoll_pwait
probe-definition(0): sys_epoll_pwait
symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Looking at the vmlinux_path (7 entries long)
Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
Try to find probe point from debuginfo.
Symbol sys_epoll_pwait address found : ffffffff8122bd40
Matched function: SyS_epoll_pwait
Failed to get call frame on 0xffffffff8122bd40
An error occurred in debuginfo analysis (-2).
Error: Failed to add events. Reason: No such file or directory (Code: -2)

The reason is caused by libdw that, if libdw is statically linked, it
can't load libebl_{arch}.so reliable.

In this case it is still possible to get the address from
/proc/kalksyms. However, perf tries that only when libdw returns
-EBADF.

This patch gives it another chance to utilize symbol table, even if
libdw returns an error code other than -EBADF.

After applying this patch:

# perf probe -nv --add sys_epoll_pwait
probe-definition(0): sys_epoll_pwait
symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Looking at the vmlinux_path (7 entries long)
Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
Try to find probe point from debuginfo.
Symbol sys_epoll_pwait address found : ffffffff8122bd40
Matched function: SyS_epoll_pwait
Failed to get call frame on 0xffffffff8122bd40
An error occurred in debuginfo analysis (-2).
Trying to use symbols.
Opening /sys/kernel/debug/tracing/kprobe_events write=1
Added new event:
Writing event: p:probe/sys_epoll_pwait _text+2276672
probe:sys_epoll_pwait (on sys_epoll_pwait)

You can now use it in all perf tools, such as:

perf record -e probe:sys_epoll_pwait -aR sleep 1

Although libdw returns an error (Failed to get call frame), perf tries
symbol table and finally gets correct address.

Signed-off-by: Wang Nan <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Brendan Gregg <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: David Ahern <[email protected]>
Cc: He Kuang <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kaixu Xia <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Zefan Li <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
---
tools/perf/util/probe-event.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index fe4941a..f07374b 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -705,9 +705,10 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
}
/* Error path : ntevs < 0 */
pr_debug("An error occurred in debuginfo analysis (%d).\n", ntevs);
- if (ntevs == -EBADF) {
- pr_warning("Warning: No dwarf info found in the vmlinux - "
- "please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
+ if (ntevs < 0) {
+ if (ntevs == -EBADF)
+ pr_warning("Warning: No dwarf info found in the vmlinux - "
+ "please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
if (!need_dwarf) {
pr_debug("Trying to use symbols.\n");
return 0;