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

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Date:	Mon, 8 Aug 2016 16:33:21 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Masami Hiramatsu <mhiramat@...nel.org>
Cc:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
	"Wangnan (F)" <wangnan0@...wei.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: perf test BPF failing on f24: fix

Em Sat, Aug 06, 2016 at 07:29:48PM +0900, Masami Hiramatsu escreveu:
> On Fri, 5 Aug 2016 11:35:32 -0300
> Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> 
> > Em Fri, Aug 05, 2016 at 06:45:50PM +0900, Masami Hiramatsu escreveu:
> > > On Thu, 4 Aug 2016 18:47:24 -0300
> > > Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > > 
> > > > Em Thu, Aug 04, 2016 at 04:36:56PM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > > So:
> > > > > 
> > > > > int err = debuginfo__get_text_offset(dbg, &baseaddr);
> > > > > 
> > > > > is returning 0, no relocation, its dwarf_addrdie() that is not finding
> > > > > SyS_epoll_wait from its address.
> > > > > 
> > > > > Trying to figure out why dwarf_addrdie(0xffffffffbd295b50) fails...
> > > > 
> > > > So, trying to use that vmlinux with objdump to do disassembly I found
> > > > that I need to do some offsetting, and after calculating it, this made
> > > > things works for me:
> > > > 
> > > > diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
> > > > index f2d9ff064e2d..9b95754f28ed 100644
> > > > --- a/tools/perf/util/probe-finder.c
> > > > +++ b/tools/perf/util/probe-finder.c
> > > > @@ -1486,6 +1486,8 @@ retry:
> > > >  	/* Find cu die */
> > > >  	if (!dwarf_addrdie(dbg->dbg, (Dwarf_Addr)addr, &cudie)) {
> > > >  		if (!reloc && debuginfo__get_text_offset(dbg, &baseaddr) == 0) {
> > > > +			if (baseaddr == 0)
> > > > +				baseaddr = -0x3c000000;
> > > 
> > > Hmm, strange... what's this magic number ...?
> > > Actually debuginfo__get_text_offset() is for kernel modules and it relocate the offset.
> 
> I've found "commit f90acac75713cc6f18a4b2f1b9162bc1cd893c20"
> 
> Actually, I forgot to count the relocation at find_alternative_probe_point().
> Could you try the patch below?
> 
> ------
> perf-probe: Adjust map->reloc offset when finding kernel symbol from map

Does the trick, booting with a kernel built from what is in
tip/perf/urgent, and with CONFIG_RANDOMIZE_BASE=y, before this fix:

[root@...et ~]# 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
Failed to get sdt note: -9
Failed to update/scan SDT cache for (null)
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 : ffffffffac296900
Failed to find debug information for address ffffffffac296900
Probe point 'sys_epoll_wait' not found.
  Error: Failed to add events. Reason: No such file or directory (Code:
-2)
[root@...et ~]# 
[root@...et ~]# 

After the fix:

[root@...et ~]# 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 : ffffffff81296900
Matched function: SyS_epoll_wait
found inline addr: 0xffffffff81296c97
Probe point found: compat_SyS_epoll_pwait+151
found inline addr: 0xffffffff81296a7a
Probe point found: SyS_epoll_pwait+138
found inline addr: 0xffffffff81296900
Probe point found: SyS_epoll_wait+0
Found 3 probe_trace_events.
Opening /sys/kernel/debug/tracing//kprobe_events write=1
Writing event: p:probe/sys_epoll_wait _text+2714775
Writing event: p:probe/sys_epoll_wait_1 _text+2714234
Writing event: p:probe/sys_epoll_wait_2 _text+2713856
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 ~]#