Messages in this thread Patch in this message | | | From | Wang Nan <> | Subject | [PATCH 01/29] perf probe: Try to use symbol table if searching debug info failed | Date | Fri, 21 Aug 2015 10:09:02 +0000 |
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A problem can occure in 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 return 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 <wangnan0@huawei.com> --- 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; -- 2.1.0
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