Re: perf test BPF failing on f24: fix

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Aug 02, 2016 at 02:03:33PM -0700, Alexei Starovoitov escreveu:
> 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@jouet ~]# grep -i sys_epoll_wait /proc/kallsyms 
> > ffffffffbd295b50 T SyS_epoll_wait
> > ffffffffbd295b50 T sys_epoll_wait
> > [root@jouet ~]#
> > 
> > I noticed that it is some sort of aliasing so I checked the other
> > variant:
> > 
> > [root@jouet ~]# 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@jouet ~]# 
> > 
> > Trying to use perf probe also produces the same resuls I notice when
> > running the perf test that is failing:
> > 
> > [root@jouet ~]# 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@jouet ~]# 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_

Is there a promise that internal kernel functions will not change names?

> 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?

Was it disabled? What I noticed whas that the sys_epoll_wait wasn't
present in the DWARF info present in the vmlinux file, which would be
good for others to confirm, which I'll check on other machines here at
home, tomorrow.

- Arnaldo
 
> > 
> > You can now use it in all perf tools, such as:
> > 
> > 	perf record -e probe:SyS_epoll_wait_2 -aR sleep 1
> > 
> > [root@jouet ~]# 
> > 
> > So I am changing the BPF perf test to use the CamelCase notation alias:
> > 
> > Failing (with sys_epoll_wait):
> > 
> > [root@jouet ~]# 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@jouet ~]#
> > 
> > Changing to (SyS_epoll_wait):
> > 
> > [root@jouet ~]# 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@jouet ~]#
> > 
> > I'm adding a fix that just makes it use SyS_epoll_wait.
> > 
> > - Arnaldo