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

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Date:	Thu, 4 Aug 2016 18:47:24 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:	Masami Hiramatsu <mhiramat@...nel.org>,
	"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 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;
 			addr += baseaddr;
 			reloc = true;
 			goto retry;

----------------

With it, which is not a proper fix, of course, we get:

[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 ~]# 

Which, using -v shows it "finding" the alias:

[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 : ffffffffbd295b50
Matched function: SyS_epoll_wait
found inline addr: 0xffffffff81295ee7
Probe point found: compat_SyS_epoll_pwait+151
found inline addr: 0xffffffff81295cca
Probe point found: SyS_epoll_pwait+138
found inline addr: 0xffffffff81295b50
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+2711271
Writing event: p:probe/sys_epoll_wait_1 _text+2710730
Writing event: p:probe/sys_epoll_wait_2 _text+2710352
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 ~]# 

----------

Now to figure out why baseaddr is returning as zero...

- Arnaldo