Re: perf test BPF failing on f24: fix

[Masami Hiramatsu <mhiramat@kernel.org>]

On Thu, 4 Aug 2016 09:50:25 +0800
"Wangnan (F)" <wangnan0@huawei.com> wrote:

> 
> 
> On 2016/8/4 7:08, Masami Hiramatsu wrote:
> > On Wed, 3 Aug 2016 23:45:57 +0900
> > Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> >>>> [root@jouet ~]# perf probe sys_epoll_wait
> >>>> Failed to find debug information for address ffffffffbd295b50
> >>>> Probe point 'sys_epoll_wait' not found.
> >>>>     Error: Failed to add events.
> >> So, at least I should investigate it.
> > FYI, on ubuntu16.04, I can put probes on sys_epoll_wait as below
> >
> > mhiramat@devnote:~/ksrc/linux/tools/perf$ sudo ./perf probe -vnf sys_epoll_wait
> > probe-definition(0): sys_epoll_wait
> > symbol:sys_epoll_wait file:(null) line:0 offset:0 return:0 lazy:(null)
> > 0 arguments
> > Looking at the vmlinux_path (8 entries long)
> > Using /usr/lib/debug/boot/vmlinux-4.4.0-31-generic for symbols
> > Open Debuginfo file: /usr/lib/debug/boot/vmlinux-4.4.0-31-generic
> > Try to find probe point from debuginfo.
> > Symbol sys_epoll_wait address found : ffffffff81257410
> > Matched function: SyS_epoll_wait
> > found inline addr: 0xffffffff81257410
> > Probe point found: SyS_epoll_wait+0
> > found inline addr: 0xffffffff8125756d
> > Probe point found: SyS_epoll_pwait+141
> > found inline addr: 0xffffffff81257793
> > Probe point found: compat_SyS_epoll_pwait+147
> > Found 3 probe_trace_events.
> > Opening /sys/kernel/debug/tracing//kprobe_events write=1
> > Writing event: p:probe/sys_epoll_wait _text+2454544
> > Writing event: p:probe/sys_epoll_wait_1 _text+2454893
> > Writing event: p:probe/sys_epoll_wait_2 _text+2455443
> > Added new events:
> >    probe:sys_epoll_wait (on sys_epoll_wait)
> >    probe:sys_epoll_wait_1 (on sys_epoll_wait)
> >    probe:sys_epoll_wait_2 (on sys_epoll_wait)
> >
> > You can now use it in all perf tools, such as:
> >
> > 	perf record -e probe:sys_epoll_wait_2 -aR sleep 1
> >
> 
> 
> Have you checked 'readelf -wi'? I guess you wouldn't find
> sys_epoll_wait.

Yes, my understandig is that the sys_* functions (alias) are
generated by linker, so debuginfo doesn't know about it.

As the above log said, perf-probe(and bpf too) tries to find
symbol in kallsyms and find the corresponding debuginfo entry(DIE)
and find the probe point again in debuginfo.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>