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

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Message-ID: <57A16807.9010901@huawei.com>
Date:	Wed, 3 Aug 2016 11:41:59 +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 10:41, Wangnan (F) wrote:
>
>
> 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.
>

Still unable reproduce. I tried tip/core/core, which is '4.7.0+'.
I also turns off CONFIG_DEBUG_INFO. In this case, testcase
BPF prologue fails as expected, and basic BPF test is okay.

After removing vmlinux, with kallsyms information
'perf probe sys_epoll_wait' sets only 1 kprobe, but it still
works.

I think there is a bug in your vmlinux. Could you please
do a bisect to find which commit causes this problem?

Thank you.