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Subject[PATCH v2 0/2] perf probe fixes for ppc64le
Date
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


Concept Acked-by: Michael Ellerman <mpe@ellerman.id.au>

Cc: Mark Wielaard <mjw@redhat.com>
Cc: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>

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

tools/perf/arch/powerpc/util/sym-handling.c | 43 +++++++++++++++++++++--------
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, 43 insertions(+), 18 deletions(-)

--
2.7.4

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