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

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Date:	Wed, 3 Aug 2016 16:19:31 +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>,
	Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: perf test BPF failing on f24: fix

Add Masami to Cc list. He knowns debug info.

Please see below...

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
> #

As I have said in another mail, although I don't know why,
I don't think the missing of sys_epoll_wait is the root cause
of your failure.

I did a bisect search and found commit 
83460ec8dcac14142e7860a01fa59c267ac4657c:
"syscalls.h: use gcc alias instead of assembler aliases for syscalls"
After this commit, sys_xxx symbols is gone from output of 'readelf -wi'. 
It is
introduced in v3.13.

> But:
>
> [root@...et ~]# grep -i sys_epoll_wait /proc/kallsyms
> ffffffffbd295b50 T SyS_ep

> oll_wait
> ffffffffbd295b50 T sys_epoll_wait
> [root@...et ~]#

If we remove vmlinux, perf should use /proc/kallsyms. I think
the failure would gone. The problem is: when symbol searching
fail using vmlinux, should we fallback to kallsyms? However,
this is another question.

> 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 ~]#
>
> 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.
> [root@...et ~]# perf probe SyS_epoll_wait
> 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
>
> [root@...et ~]#
>
> So I am changing the BPF perf test to use the CamelCase notation alias:

Changing to SyS_xxx is okay, byt we still need to know the root
cause.

Thank you.