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

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Message-ID: <20160802210331.GA20688@ast-mbp.thefacebook.com>
Date:	Tue, 2 Aug 2016 14:03:33 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Wang Nan <wangnan0@...wei.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	brendan.d.gregg@...il.com, bblanco@...mgrid.com
Subject: Re: perf test BPF failing on f24: fix

On Tue, Aug 02, 2016 at 04:51:02PM -0300, 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 ~]# 
> 
> 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)

that change will break all sorts of scripts that relying on syscalls to start
with sys_
I guess we can workaround in user space, but what was the motivation to
disable kprobe attach on sys_* while it's still in kallsyms?

> 
> 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:
> 
> Failing (with sys_epoll_wait):
> 
> [root@...et ~]# perf test BPF
> 37: Test BPF filter                                          :
> 37.1: Test basic BPF filtering                               : FAILED!
> 37.2: Test BPF prologue generation                           : Skip
> 37.3: Test BPF relocation checker                            : Skip
> [root@...et ~]#
> 
> Changing to (SyS_epoll_wait):
> 
> [root@...et ~]# perf test BPF
> 37: Test BPF filter                                          :
> 37.1: Test basic BPF filtering                               : Ok
> 37.2: Test BPF prologue generation                           : Ok
> 37.3: Test BPF relocation checker                            : Ok
> [root@...et ~]#
> 
> I'm adding a fix that just makes it use SyS_epoll_wait.
> 
> - Arnaldo