perf tools powerpc: 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. We should use the DWARF debug information to determine
when the entries are  needed.

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 the DWARF debug information to determine if any callchain entries
should be ignored when building call-graphs.

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.

Fix suggested by Anton Blanchard.

Thanks to valuable input on DWARF debug information from Ulrich Weigand.

Reported-by: Maynard Johnson <maynard@us.ibm.com>
Tested-by: Maynard Johnson <maynard@us.ibm.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20140625154903.GA29607@us.ibm.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>

5 files changed, 300 insertions, 2 deletions

diff --git a/tools/perf/arch/powerpc/Makefile b/tools/perf/arch/powerpc/Makefile
index 744e629797be..b92219b1900d 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/skip-callchain-idx.o
diff --git a/tools/perf/arch/powerpc/util/skip-callchain-idx.c b/tools/perf/arch/powerpc/util/skip-callchain-idx.c
new file mode 100644
index 000000000000..a7c23a4b3778
--- /dev/null
+++ b/tools/perf/arch/powerpc/util/skip-callchain-idx.c
@@ -0,0 +1,266 @@
+/*
+ * 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);
+        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)
+{
+        int             rc = -1;
+        Dwfl            *dwfl;
+        Dwfl_Module     *mod;
+        Dwarf_Frame     *frame;
+        int             ra_regno;
+        Dwarf_Addr      start = pc;
+        Dwarf_Addr      end = pc;
+        bool            signalp;
+
+        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));
+                goto out;
+        }
+
+        mod = dwfl_addrmodule(dwfl, pc);
+        if (!mod) {
+                pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1));
+                goto out;
+        }
+
+        /*
+         * 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)
+                        goto out;
+        }
+
+        ra_regno = dwarf_frame_info(frame, &start, &end, &signalp);
+        if (ra_regno < 0) {
+                pr_debug("Return address register unavailable: %s\n",
+                                dwarf_errmsg(-1));
+                goto out;
+        }
+
+        rc = check_return_reg(ra_regno, frame);
+
+out:
+        dwfl_end(dwfl);
+        return rc;
+}
+
+/*
+ * 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
+ */
+int arch_skip_callchain_idx(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 f30ac5e5d271..346bdb617544 100644
--- a/tools/perf/config/Makefile
+++ b/tools/perf/config/Makefile
@@ -48,6 +48,10 @@ ifneq ($(ARCH),$(filter $(ARCH),x86 arm))
   NO_LIBDW_DWARF_UNWIND := 1
 endif
 
+ifeq ($(ARCH),powerpc)
+  CFLAGS += -DHAVE_SKIP_CALLCHAIN_IDX
+endif
+
 ifeq ($(LIBUNWIND_LIBS),)
   NO_LIBUNWIND := 1
 else
diff --git a/tools/perf/util/callchain.h b/tools/perf/util/callchain.h
index 8f84423a75da..da43619d6173 100644
--- a/tools/perf/util/callchain.h
+++ b/tools/perf/util/callchain.h
@@ -176,4 +176,17 @@ static inline void callchain_cursor_snapshot(struct callchain_cursor *dest,
         dest->first = src->curr;
         dest->nr -= src->pos;
 }
+
+#ifdef HAVE_SKIP_CALLCHAIN_IDX
+extern int arch_skip_callchain_idx(struct machine *machine,
+                        struct thread *thread, struct ip_callchain *chain);
+#else
+static inline int arch_skip_callchain_idx(struct machine *machine __maybe_unused,
+                        struct thread *thread __maybe_unused,
+                        struct ip_callchain *chain __maybe_unused)
+{
+        return -1;
+}
+#endif
+
 #endif  /* __PERF_CALLCHAIN_H */
diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index c73e1fc12e53..e9b943acaa5e 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -1281,7 +1281,9 @@ static int machine__resolve_callchain_sample(struct machine *machine,
         u8 cpumode = PERF_RECORD_MISC_USER;
         int chain_nr = min(max_stack, (int)chain->nr);
         int i;
+        int j;
         int err;
+        int skip_idx __maybe_unused;
 
         callchain_cursor_reset(&callchain_cursor);
 
@@ -1290,14 +1292,26 @@ static int machine__resolve_callchain_sample(struct machine *machine,
                 return 0;
         }
 
+        /*
+         * Based on DWARF debug information, some architectures skip
+         * a callchain entry saved by the kernel.
+         */
+        skip_idx = arch_skip_callchain_idx(machine, thread, chain);
+
         for (i = 0; i < chain_nr; i++) {
                 u64 ip;
                 struct addr_location al;
 
                 if (callchain_param.order == ORDER_CALLEE)
-                        ip = chain->ips[i];
+                        j = i;
                 else
-                        ip = chain->ips[chain->nr - i - 1];
+                        j = chain->nr - i - 1;
+
+#ifdef HAVE_SKIP_CALLCHAIN_IDX
+                if (j == skip_idx)
+                        continue;
+#endif
+                ip = chain->ips[j];
 
                 if (ip >= PERF_CONTEXT_MAX) {
                         switch (ip) {