linux-kernel - [PATCH 0/2] perf probe fixes for ppc64le

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Message-Id: <cover.1459942081.git.naveen.n.rao@linux.vnet.ibm.com>
Date:	Wed,  6 Apr 2016 18:02:56 +0530
From:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc:	Mark Wielaard <mjw@...hat.com>,
	Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: [PATCH 0/2] perf probe fixes for ppc64le

This patchset fixes three issues found with perf probe on ppc64le:
1. 'perf test kallsyms' failure on ppc64le (reported by Michael
Ellerman). This was due to the symbols being fixed up during symbol
table load. This is fixed in patch 2 by delaying symbol fixup until
later.
2. perf probe function offset was being calculated from the local entry
point (LEP), which does not match user expectation when trying to look
at function disassembly output (reported by Ananth N). This is fixed for
kallsyms in patch 1 and for symbol table in patch 2.
3. perf probe failure with kretprobe when using kallsyms. This was
failing as we were specifying an offset. This is fixed in patch 1.

A few examples demonstrating the issues and the fix:

Example for issue (2):
--------------------
    # objdump -d vmlinux | grep -A8 \<_do_fork\>:
    c0000000000b6a00 <_do_fork>:
    c0000000000b6a00:	f7 00 4c 3c 	addis   r2,r12,247
    c0000000000b6a04:	00 86 42 38 	addi    r2,r2,-31232
    c0000000000b6a08:	a6 02 08 7c 	mflr    r0
    c0000000000b6a0c:	d0 ff 41 fb 	std     r26,-48(r1)
    c0000000000b6a10:	26 80 90 7d 	mfocrf  r12,8
    c0000000000b6a14:	d8 ff 61 fb 	std     r27,-40(r1)
    c0000000000b6a18:	e0 ff 81 fb 	std     r28,-32(r1)
    c0000000000b6a1c:	e8 ff a1 fb 	std     r29,-24(r1)
    # perf probe -v _do_fork+4
    probe-definition(0): _do_fork+4 
    symbol:_do_fork file:(null) line:0 offset:4 return:0 lazy:(null)
    0 arguments
    Looking at the vmlinux_path (8 entries long)
    Using /proc/kcore for kernel object code
    Using /proc/kallsyms for symbols
    Opening /sys/kernel/debug/tracing//kprobe_events write=1
    Writing event: p:probe/_do_fork _text+748044
    Added new event:
      probe:_do_fork       (on _do_fork+4)

    You can now use it in all perf tools, such as:

	    perf record -e probe:_do_fork -aR sleep 1

    # printf "%x\n" 748044
    b6a0c
    ^^^^^
This is offset from the LEP. With this, there is also no way to ever
probe between the GEP and the LEP.

With this patchset:
    # perf probe -v _do_fork+4
    probe-definition(0): _do_fork+4 
    symbol:_do_fork file:(null) line:0 offset:4 return:0 lazy:(null)
    0 arguments
    Looking at the vmlinux_path (8 entries long)
    Using /proc/kcore for kernel object code
    Using /proc/kallsyms for symbols
    Opening /sys/kernel/debug/tracing//kprobe_events write=1
    Writing event: p:probe/_do_fork _text+748036
    Added new event:
      probe:_do_fork       (on _do_fork+4)

    You can now use it in all perf tools, such as:

	    perf record -e probe:_do_fork -aR sleep 1

    # perf probe -v _do_fork
    probe-definition(0): _do_fork 
    symbol:_do_fork file:(null) line:0 offset:0 return:0 lazy:(null)
    0 arguments
    Looking at the vmlinux_path (8 entries long)
    Using /proc/kcore for kernel object code
    Using /proc/kallsyms for symbols
    Opening /sys/kernel/debug/tracing//kprobe_events write=1
    Writing event: p:probe/_do_fork _text+748040
    Added new event:
      probe:_do_fork       (on _do_fork)

    You can now use it in all perf tools, such as:

	    perf record -e probe:_do_fork -aR sleep 1

We only offset to the LEP if function entry is specified, otherwise, we
offset from the GEP.

Example for issue (3):
---------------------
Before patch:
    # perf probe -v _do_fork:%return
    probe-definition(0): _do_fork:%return 
    symbol:_do_fork file:(null) line:0 offset:0 return:1 lazy:(null)
    0 arguments
    Looking at the vmlinux_path (8 entries long)
    Using /proc/kcore for kernel object code
    Using /proc/kallsyms for symbols
    Opening /sys/kernel/debug/tracing//kprobe_events write=1
    Writing event: r:probe/_do_fork _do_fork+8
    Failed to write event: Invalid argument
      Error: Failed to add events. Reason: Invalid argument (Code: -22)

After patch:
    # perf probe _do_fork:%return
    Added new event:
      probe:_do_fork       (on _do_fork%return)

    You can now use it in all perf tools, such as:

	    perf record -e probe:_do_fork -aR sleep 1

Cc: Mark Wielaard <mjw@...hat.com>
Cc: Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Ananth N Mavinakayanahalli <ananth@...ibm.com>

Naveen N. Rao (2):
  perf/powerpc: Fix kprobe and kretprobe handling with kallsyms
  tools/perf: Fix kallsyms perf test on ppc64le

 tools/perf/arch/powerpc/util/sym-handling.c | 41 ++++++++++++++++++++---------
 tools/perf/util/probe-event.c               |  5 ++--
 tools/perf/util/probe-event.h               |  3 ++-
 tools/perf/util/symbol-elf.c                |  7 ++---
 tools/perf/util/symbol.h                    |  3 ++-
 5 files changed, 40 insertions(+), 19 deletions(-)

-- 
2.7.4