Sukadev Bhattiprolu
2014-05-10T02:46:47Z

[PATCH 1/1] powerpc/perf: Adjust callchain based on DWARF debug info

When saving the callchain on Power, the kernel conservatively saves excess
entries in the callchain. A few of these entries are needed in some cases
but not others.

Eg: the value in the link register (LR) is needed only when it holds the
return address of a function. At other times it must be ignored.

If the unnecessary entries are not ignored, we end up with duplicate arcs
in the call-graphs.

Use DWARF debug information to ignore the unnecessary entries.

Callgraph before the patch:

14.67% 2234 sprintft libc-2.18.so [.] __random
|
--- __random
|
|--61.12%-- __random
| |
| |--97.15%-- rand
| | do_my_sprintf
| | main
| | generic_start_main.isra.0
| | __libc_start_main
| | 0x0
| |
| --2.85%-- do_my_sprintf
| main
| generic_start_main.isra.0
| __libc_start_main
| 0x0
|
--38.88%-- rand
|
|--94.01%-- rand
| do_my_sprintf
| main
| generic_start_main.isra.0
| __libc_start_main
| 0x0
|
--5.99%-- do_my_sprintf
main
generic_start_main.isra.0
__libc_start_main
0x0

Callgraph after the patch:

14.67% 2234 sprintft libc-2.18.so [.] __random
|
--- __random
|
|--95.93%-- rand
| do_my_sprintf
| main
| generic_start_main.isra.0
| __libc_start_main
| 0x0
|
--4.07%-- do_my_sprintf
main
generic_start_main.isra.0
__libc_start_main
0x0

TODO: For split-debug info objects like glibc, we can only determine
the call-frame-address only when both .eh_frame and .debug_info
sections are available. We should be able to determin the CFA
even without the .eh_frame section.

Thanks to Ulrich Weigand for help with DWARF debug information.

Fix suggested by Anton Blanchard.

Reported-by: Maynard Johnson <maynard@us.ibm.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
---
tools/perf/arch/powerpc/Makefile | 1 +
tools/perf/arch/powerpc/util/adjust-callchain.c | 278 ++++++++++++++++++++++++
tools/perf/config/Makefile | 5 +
tools/perf/util/callchain.h | 12 +
tools/perf/util/machine.c | 16 +-
5 files changed, 310 insertions(+), 2 deletions(-)
create mode 100644 tools/perf/arch/powerpc/util/adjust-callchain.c

diff --git a/tools/perf/arch/powerpc/Makefile b/tools/perf/arch/powerpc/Makefile
index 744e629..512cc8d 100644
--- a/tools/perf/arch/powerpc/Makefile
+++ b/tools/perf/arch/powerpc/Makefile
@@ -3,3 +3,4 @@ PERF_HAVE_DWARF_REGS := 1
LIB_OBJS += $(OUTPUT)arch/$(ARCH)/util/dwarf-regs.o
endif
LIB_OBJS += $(OUTPUT)arch/$(ARCH)/util/header.o
+LIB_OBJS += $(OUTPUT)arch/$(ARCH)/util/adjust-callchain.o
diff --git a/tools/perf/arch/powerpc/util/adjust-callchain.c b/tools/perf/arch/powerpc/util/adjust-callchain.c
new file mode 100644
index 0000000..31b1f95
--- /dev/null
+++ b/tools/perf/arch/powerpc/util/adjust-callchain.c
@@ -0,0 +1,278 @@
+/*
+ * Use DWARF Debug information to skip unnecessary callchain entries.
+ *
+ * Copyright (C) 2014 Sukadev Bhattiprolu, IBM Corporation.
+ * Copyright (C) 2014 Ulrich Weigand, IBM Corporation.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#include <inttypes.h>
+#include <dwarf.h>
+#include <elfutils/libdwfl.h>
+
+#include "util/thread.h"
+#include "util/callchain.h"
+
+/*
+ * When saving the callchain on Power, the kernel conservatively saves
+ * excess entries in the callchain. A few of these entries are needed
+ * in some cases but not others. If the unnecessary entries are not
+ * ignored, we end up with duplicate arcs in the call-graphs. Use
+ * DWARF debug information to skip over any unnecessary callchain
+ * entries.
+ *
+ * See function header for arch_adjust_callchain() below for more details.
+ *
+ * The libdwfl code in this file is based on code from elfutils
+ * (libdwfl/argp-std.c, libdwfl/tests/addrcfi.c, etc).
+ */
+static char *debuginfo_path;
+
+static const Dwfl_Callbacks offline_callbacks = {
+ .debuginfo_path = &debuginfo_path,
+ .find_debuginfo = dwfl_standard_find_debuginfo,
+ .section_address = dwfl_offline_section_address,
+};
+
+
+/*
+ * Use the DWARF expression for the Call-frame-address and determine
+ * if return address is in LR and if a new frame was allocated.
+ */
+static int check_return_reg(int ra_regno, Dwarf_Frame *frame)
+{
+ Dwarf_Op ops_mem[2];
+ Dwarf_Op dummy;
+ Dwarf_Op *ops = &dummy;
+ size_t nops;
+ int result;
+
+ result = dwarf_frame_register(frame, ra_regno, ops_mem, &ops, &nops);
+ if (result < 0) {
+ pr_debug("dwarf_frame_register() %s\n", dwarf_errmsg(-1));
+ return -1;
+ }
+
+ /*
+ * Check if return address is on the stack.
+ */
+ if (nops != 0 || ops != NULL)
+ return 0;
+
+ /*
+ * Return address is in LR. Check if a frame was allocated
+ * but not-yet used.
+ */
+ result = dwarf_frame_cfa(frame, &ops, &nops);
+ if (result < 0) {
+ pr_debug("dwarf_frame_cfa() returns %d, %s\n", result,
+ dwarf_errmsg(-1));
+ return -1;
+ }
+
+ /*
+ * If call frame address is in r1, no new frame was allocated.
+ */
+ if (nops == 1 && ops[0].atom == DW_OP_bregx && ops[0].number == 1 &&
+ ops[0].number2 == 0)
+ return 1;
+
+ /*
+ * A new frame was allocated but has not yet been used.
+ */
+ return 2;
+}
+
+/*
+ * Get the DWARF frame from the .eh_frame section.
+ */
+static Dwarf_Frame *get_eh_frame(Dwfl_Module *mod, Dwarf_Addr pc)
+{
+ int result;
+ Dwarf_Addr bias;
+ Dwarf_CFI *cfi;
+ Dwarf_Frame *frame;
+
+ cfi = dwfl_module_eh_cfi(mod, &bias); // CHECK
+ if (!cfi) {
+ pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
+ return NULL;
+ }
+
+ result = dwarf_cfi_addrframe(cfi, pc, &frame);
+ if (result) {
+ pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
+ return NULL;
+ }
+
+ return frame;
+}
+
+/*
+ * Get the DWARF frame from the .debug_frame section.
+ */
+static Dwarf_Frame *get_dwarf_frame(Dwfl_Module *mod, Dwarf_Addr pc)
+{
+ Dwarf_CFI *cfi;
+ Dwarf_Addr bias;
+ Dwarf_Frame *frame;
+ int result;
+
+ cfi = dwfl_module_dwarf_cfi(mod, &bias);
+ if (!cfi) {
+ pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
+ return NULL;
+ }
+
+ result = dwarf_cfi_addrframe(cfi, pc, &frame);
+ if (result) {
+ pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
+ return NULL;
+ }
+
+ return frame;
+}
+
+/*
+ * Return:
+ * 0 if return address for the program counter @pc is on stack
+ * 1 if return address is in LR and no new stack frame was allocated
+ * 2 if return address is in LR and a new frame was allocated (but not
+ * yet used)
+ * -1 in case of errors
+ */
+static int check_return_addr(const char *exec_file, Dwarf_Addr pc)
+{
+ Dwfl *dwfl;
+ Dwfl_Module *mod;
+ Dwarf_Frame *frame;
+ int ra_regno;
+ Dwarf_Addr start = pc;
+ Dwarf_Addr end = pc;
+ bool signalp;
+
+ pr_debug("Testing: %s, @0x%llx\n", exec_file, (unsigned long long)pc);
+
+ dwfl = dwfl_begin(&offline_callbacks);
+ if (!dwfl) {
+ pr_debug("dwfl_begin() failed: %s\n", dwarf_errmsg(-1));
+ return -1;
+ }
+
+ if (dwfl_report_offline(dwfl, "", exec_file, -1) == NULL) {
+ pr_debug("dwfl_report_offline() failed %s\n", dwarf_errmsg(-1));
+ return -1;
+ }
+
+ mod = dwfl_addrmodule(dwfl, pc);
+ if (!mod) {
+ pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1));
+ return -1;
+ }
+
+ /*
+ * To work with split debug info files (eg: glibc), check both
+ * .eh_frame and .debug_frame sections of the ELF header.
+ */
+ frame = get_eh_frame(mod, pc);
+ if (!frame) {
+ frame = get_dwarf_frame(mod, pc);
+ if (!frame)
+ return -1;
+ }
+
+ ra_regno = dwarf_frame_info (frame, &start, &end, &signalp);
+ if (ra_regno < 0) {
+ pr_debug("Return address register unavailable: %s\n",
+ dwarf_errmsg(-1));
+ return -1;
+ }
+
+ return check_return_reg(ra_regno, frame);
+}
+
+/*
+ * The callchain saved by the kernel always includes the link register (LR).
+ *
+ * 0: PERF_CONTEXT_USER
+ * 1: Program counter (Next instruction pointer)
+ * 2: LR value
+ * 3: Caller's caller
+ * 4: ...
+ *
+ * The value in LR is only needed when it holds a return address. If the
+ * return address is on the stack, we should ignore the LR value.
+ *
+ * Further, when the return address is in the LR, if a new frame was just
+ * allocated but the LR was not saved into it, then the LR contains the
+ * caller, slot 4: contains the caller's caller and the contents of slot 3:
+ * (chain->ips[3]) is undefined and must be ignored.
+ *
+ * Use DWARF debug information to determine if any entries need to be skipped.
+ *
+ * Return:
+ * index: of callchain entry that needs to be ignored (if any)
+ * -1 if no entry needs to be ignored or in case of errors
+ *
+ * TODO:
+ * Rather than returning an index into the callchain and have the
+ * caller skip that entry, we could modify the callchain in-place
+ * by putting a PERF_CONTEXT_IGNORE marker in the affected entry.
+ *
+ * But @chain points to read-only mmap, so the caller needs to
+ * duplicate the callchain to modify in-place - something like:
+ *
+ * new_callchain = arch_duplicate_callchain()
+ * arch_adjust_callchain(new_callchain)
+ * arch_free_callchain(new_callchain)
+ *
+ * Since we only expect to adjust <= 1 entry for now, just return
+ * the index.
+ */
+int arch_adjust_callchain(struct machine *machine, struct thread *thread,
+ struct ip_callchain *chain)
+{
+ struct addr_location al;
+ struct dso *dso = NULL;
+ int rc;
+ u64 ip;
+ u64 skip_slot = -1;
+
+ if (chain->nr < 3)
+ return skip_slot;
+
+ ip = chain->ips[2];
+
+ thread__find_addr_location(thread, machine, PERF_RECORD_MISC_USER,
+ MAP__FUNCTION, ip, &al);
+
+ if (al.map)
+ dso = al.map->dso;
+
+ if (!dso) {
+ pr_debug("%" PRIx64 " dso is NULL\n", ip);
+ return skip_slot;
+ }
+
+ rc = check_return_addr(dso->long_name, ip);
+
+ pr_debug("DSO %s, nr %" PRIx64 ", ip 0x%" PRIx64 "rc %d\n",
+ dso->long_name, chain->nr, ip, rc);
+
+ if (rc == 0) {
+ /*
+ * Return address on stack. Ignore LR value in callchain
+ */
+ skip_slot = 2;
+ } else if (rc == 2) {
+ /*
+ * New frame allocated but return address still in LR.
+ * Ignore the caller's caller entry in callchain.
+ */
+ skip_slot = 3;
+ }
+ return skip_slot;
+}
diff --git a/tools/perf/config/Makefile b/tools/perf/config/Makefile
index 5a3c452..7e93877 100644
--- a/tools/perf/config/Makefile
+++ b/tools/perf/config/Makefile
@@ -29,11 +29,16 @@ ifeq ($(ARCH),x86)
endif
NO_PERF_REGS := 0
endif
+
ifeq ($(ARCH),arm)
NO_PERF_REGS := 0
LIBUNWIND_LIBS = -lunwind -lunwind-arm
endif

+ifeq ($(ARCH),powerpc)
+ CFLAGS += -DHAVE_ADJUST_CALLCHAIN
+endif
+
ifeq ($(LIBUNWIND_LIBS),)
NO_LIBUNWIND := 1
else
diff --git a/tools/perf/util/callchain.h b/tools/perf/util/callchain.h
index 8ad97e9..81ecb90 100644
--- a/tools/perf/util/callchain.h
+++ b/tools/perf/util/callchain.h
@@ -157,4 +157,16 @@ int sample__resolve_callchain(struct perf_sample *sample, struct symbol **parent
int hist_entry__append_callchain(struct hist_entry *he, struct perf_sample *sample);

extern const char record_callchain_help[];
+
+#ifdef HAVE_ADJUST_CALLCHAIN
+extern int arch_adjust_callchain(struct machine *machine,
+ struct thread *thread, struct ip_callchain *chain);
+#else
+static inline int arch_adjust_callchain(struct machine *machine,
+ struct thread *thread, struct ip_callchain *chain)
+{
+ return -1;
+}
+#endif
+
#endif /* __PERF_CALLCHAIN_H */
diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index a53cd0b..dce3bf0 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -1271,6 +1271,7 @@ static int machine__resolve_callchain_sample(struct machine *machine,
int chain_nr = min(max_stack, (int)chain->nr);
int i;
int err;
+ int skip_slot;

callchain_cursor_reset(&callchain_cursor);

@@ -1279,14 +1280,25 @@ static int machine__resolve_callchain_sample(struct machine *machine,
return 0;
}

+ /*
+ * Based on DWARF debug information, some architectures skip
+ * some of the callchain entries saved by the kernel.
+ */
+ skip_slot = arch_adjust_callchain(machine, thread, chain);
+
for (i = 0; i < chain_nr; i++) {
u64 ip;
struct addr_location al;

- if (callchain_param.order == ORDER_CALLEE)
+ if (callchain_param.order == ORDER_CALLEE) {
+ if (i == skip_slot)
+ continue;
ip = chain->ips[i];
- else
+ } else {
+ if ((int)(chain->nr - i - 1) == skip_slot)
+ continue;
ip = chain->ips[chain->nr - i - 1];
+ }

if (ip >= PERF_CONTEXT_MAX) {
switch (ip) {
--
1.8.4.2