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

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Date:	Wed, 3 Aug 2016 23:45:57 +0900
From:	Masami Hiramatsu <mhiramat@...nel.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: perf test BPF failing on f24: fix

On Wed, 3 Aug 2016 16:19:31 +0800
"Wangnan (F)" <wangnan0@...wei.com> wrote:

> 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.

As below result shown, perf probe tries to fallback, but it seems to
fail to find corresponding debuginfo entry...

> > [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.

So, at least I should investigate it.

Thanks,

> > [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.
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>