linux-kernel - Re: perf test BPF failing on f24: fix

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Message-ID: <57A159EF.7010106@huawei.com>
Date:	Wed, 3 Aug 2016 10:41:51 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: perf test BPF failing on f24: fix

On 2016/8/3 3:51, Arnaldo Carvalho de Melo wrote:
> Hi Wang,
>
> 	Something changed and a function used in a perf test for BPF is
> not anymore appearing on vmlinux, albeit still available on
> /proc/kallsyms:
>
> # readelf -wi /lib/modules/4.7.0+/build/vmlinux | grep -w sys_epoll_wait
> #
>
> But:
>
> [root@...et ~]# grep -i sys_epoll_wait /proc/kallsyms
> ffffffffbd295b50 T SyS_epoll_wait
> ffffffffbd295b50 T sys_epoll_wait
> [root@...et ~]#
>
> I noticed that it is some sort of aliasing so I checked the other
> variant:
>
> [root@...et ~]# readelf -wi /lib/modules/4.7.0+/build/vmlinux | grep -w SyS_epoll_wait
>      <2bc9b85>   DW_AT_name        : (indirect string, offset: 0xe7524): SyS_epoll_wait
> [root@...et ~]#

I also see no sys_epoll_wait in output of 'readelf -wi', but I can't
reproduce the whole problem.

> Trying to use perf probe also produces the same resuls I notice when
> running the perf test that is failing:
>
> [root@...et ~]# 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.

For me:

# uname -r
4.7.0

# perf probe -v 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 /lib/modules/4.7.0/build/vmlinux for symbols
Open Debuginfo file: /lib/modules/4.7.0/build/vmlinux
Try to find probe point from debuginfo.
Symbol sys_epoll_wait address found : ffffffff81281240
Matched function: SyS_epoll_wait
found inline addr: 0xffffffff81281240
Probe point found: SyS_epoll_wait+0
found inline addr: 0xffffffff812813a7
Probe point found: SyS_epoll_pwait+135
found inline addr: 0xffffffff8128157a
Probe point found: compat_SyS_epoll_pwait+154
Found 3 probe_trace_events.
Opening /sys/kernel/debug/tracing//kprobe_events write=1
Writing event: p:probe/sys_epoll_wait _text+2626112
Writing event: p:probe/sys_epoll_wait_1 _text+2626471
Writing event: p:probe/sys_epoll_wait_2 _text+2626938
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

I'll try different kernel version and kconfig.

Thank you.