Re: [PATCH 1/3] perf probe: Set retprobe flag when probe in address-based alternative mode
From: He Kuang
Date: Mon Apr 13 2015 - 11:22:47 EST
On 04/13/2015 10:39 PM, Arnaldo Carvalho de Melo wrote:
Em Mon, Apr 13, 2015 at 07:41:28PM +0800, He Kuang escreveu:
Perf probe misses to set retprobe flag back when falling back to
address-based alternative mode.
And when is that happens, can you explain? Because I tried to follow your
instructions, but in my case it worked in the same way before and after your
patch.
Can you please provide more context for reviewing your patch?
More details:
[root]# perf probe -v -k vmlinux --add='sys_write%return'
probe-definition(0): sys_write%return
symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
0 arguments
Use vmlinux: vmlinux
map_groups__set_modules_path_dir: cannot open /lib/modules/4.0.0-rc6+ dir
Problems setting modules path maps, continuing anyway...
Using vmlinux for symbols
Open Debuginfo file: vmlinux
>>>==========================
Try to find probe point from debuginfo.
Symbol sys_write address found : ffffffff811a8cf0
Probe point found: SyS_write+0
Found 1 probe_trace_events.
Opening /sys/kernel/debug/tracing/kprobe_events write=1
Added new event:
Writing event: p:probe/sys_write _stext+1739560
probe:sys_write (on sys_write%return)
You can now use it in all perf tools, such as:
perf record -e probe:sys_write -aR sleep 1
[root@buildroot tmp]# cat /sys/kernel/debug/tracing/kprobe_events
p:probe/sys_write _stext+1739560
The line I marked is different from your result, which is:
"""Could not open debuginfo. Try to use symbols."""
When perf probe searched in debuginfo and failed, it tried with
alternative, in function get_alternative_probe_event():
memcpy(tmp, &pev->point, sizeof(*tmp));
memset(&pev->point, 0, sizeof(pev->point));
In this case, it drops the retprobe flag and forgets to set it back in
find_alternative_probe_point(), so the problem occurred.
Here are my results:
Before:
[root@ssdandy ~]# perf probe -v --add='sys_write%return'
probe-definition(0): sys_write%return
symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
0 arguments
Using /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda for symbols
Could not open debuginfo. Try to use symbols.
Opening /sys/kernel/debug/tracing/kprobe_events write=1
Added new event:
Writing event: r:probe/sys_write sys_write+0
probe:sys_write (on sys_write%return)
You can now use it in all perf tools, such as:
perf record -e probe:sys_write -aR sleep 1
[root@ssdandy ~]# cat /sys/kernel/debug/tracing/kprobe_events
r:probe/sys_write sys_write
Remove it:
[root@ssdandy ~]# perf probe --del *:*
Removed event: probe:sys_write
After:
[root@ssdandy ~]# perf probe -v --add='sys_write%return'
probe-definition(0): sys_write%return
symbol:sys_write file:(null) line:0 offset:0 return:1 lazy:(null)
0 arguments
Using /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda for symbols
Could not open debuginfo. Try to use symbols.
Opening /sys/kernel/debug/tracing/kprobe_events write=1
Added new event:
Writing event: r:probe/sys_write sys_write+0
probe:sys_write (on sys_write%return)
You can now use it in all perf tools, such as:
perf record -e probe:sys_write -aR sleep 1
[root@ssdandy ~]#
[root@ssdandy ~]# cat /sys/kernel/debug/tracing/kprobe_events
r:probe/sys_write sys_write
Humm, noticed one other problem, but not with your patch, about this message:
"Could not open debuginfo. Try to use symbols."
That is really not clear, specially that "try to use symbols" :-)
[root@ssdandy ~]# ls -la /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda
lrwxrwxrwx. 1 root root 86 Apr 10 18:02 /root/.debug/.build-id/dd/32e51921ede0fd46f034091b7f6a0f2e01ebda -> ../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
[root@ssdandy ~]# ls -la /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
-rwxr-xr-x. 1 root root 22698661 Apr 10 18:02 /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
[root@ssdandy ~]# ls -la /root/.debug/.build-id/dd/../../home/acme/git/build/v4.0.0-rc6+/vmlinux/dd32e51921ede0fd46f034091b7f6a0f2e01ebda
I.e. it managed to read the debuginfo, its just that it has no symbols in it :-)
Anyway, digression ended.
- Arnaldo
Can be reproduced as following:
$ perf probe -v -k vmlinux --add='sys_write%return'
...
Added new event:
Writing event: p:probe/sys_write _stext+1584952
probe:sys_write (on sys_write%return)
$ cat /sys/kernel/debug/tracing/kprobe_events
p:probe/sys_write _stext+1584952
After this patch:
$ perf probe -v -k vmlinux --add='sys_write%return'
Added new event:
Writing event: r:probe/sys_write SyS_write+0
probe:sys_write (on sys_write%return)
$ cat /sys/kernel/debug/tracing/kprobe_events
r:probe/sys_write SyS_write
Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
---
tools/perf/util/probe-event.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 30545ce..5483d98 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -332,6 +332,7 @@ static int find_alternative_probe_point(struct debuginfo *dinfo,
else {
result->offset += pp->offset;
result->line += pp->line;
+ result->retprobe = pp->retprobe;
ret = 0;
}
--
2.3.3.220.g9ab698f
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