Re: [PATCH v5 6/6] perf arm64: inject missing frames if perf-record used "--call-graph=fp"

[James Clark]

On 17/12/2021 15:45, German Gomez wrote:
> From: Alexandre Truong <alexandre.truong@xxxxxxx>
> 
> When unwinding using frame pointers on ARM64, the return address of the
> current function may not have been pushed into the stack when a function
> was interrupted, which makes perf show an incorrect call graph to the
> user.
> 
> Consider the following example program:
> 
>   void leaf() {
>       /* long computation */
>   }
> 
>   void parent() {
>       // (1)
>       leaf();
>       // (2)
>   }
> 
>   ... could be compiled into (using gcc -fno-inline -fno-omit-frame-pointer):
> 
>   leaf:
>       /* long computation */
>       nop
>       ret
>   parent:
>       // (1)
>       stp     x29, x30, [sp, -16]!
>       mov     x29, sp
>       bl      parent
>       nop
>       ldp     x29, x30, [sp], 16
>       // (2)
>       ret
> 
> If the program is interrupted at (1), (2), or any point in "leaf:", the
> call graph will skip the callers of the current function. We can unwind
> using the dwarf info and check if the return addr is the same as the LR
> register, and inject the missing frame into the call graph.
> 
> Before this patch, the above example shows the following call-graph when
> recording using "--call-graph fp" mode in ARM64:
> 
>   # Children      Self  Command   Shared Object     Symbol
>   # ........  ........  ........  ................  ......................
>   #
>       99.86%    99.86%  program3  program3          [.] leaf
>   	    |
>   	    ---_start
>   	       __libc_start_main
>   	       main
>   	       leaf
> 
> As can be seen, the "parent" function is missing. This is specially
> problematic in "leaf" because for leaf functions the compiler may always
> omit pushing the return addr into the stack. After this patch, it shows
> the correct graph:
> 
>   # Children      Self  Command   Shared Object     Symbol
>   # ........  ........  ........  ................  ......................
>   #
>       99.86%    99.86%  program3  program3          [.] leaf
>   	    |
>   	    ---_start
>   	       __libc_start_main
>   	       main
>   	       parent
>   	       leaf
> 
> Signed-off-by: Alexandre Truong <alexandre.truong@xxxxxxx>
> Signed-off-by: German Gomez <german.gomez@xxxxxxx>
> ---
>  tools/perf/util/Build                         |  1 +
>  .../util/arm64-frame-pointer-unwind-support.c | 63 +++++++++++++++++++
>  .../util/arm64-frame-pointer-unwind-support.h | 10 +++
>  tools/perf/util/machine.c                     | 19 ++++--
>  tools/perf/util/machine.h                     |  1 +
>  5 files changed, 89 insertions(+), 5 deletions(-)
>  create mode 100644 tools/perf/util/arm64-frame-pointer-unwind-support.c
>  create mode 100644 tools/perf/util/arm64-frame-pointer-unwind-support.h
> 
> diff --git a/tools/perf/util/Build b/tools/perf/util/Build
> index 2e5bfbb69960..03d4c647bd86 100644
> --- a/tools/perf/util/Build
> +++ b/tools/perf/util/Build
> @@ -1,3 +1,4 @@
> +perf-y += arm64-frame-pointer-unwind-support.o
>  perf-y += annotate.o
>  perf-y += block-info.o
>  perf-y += block-range.o
> diff --git a/tools/perf/util/arm64-frame-pointer-unwind-support.c b/tools/perf/util/arm64-frame-pointer-unwind-support.c
> new file mode 100644
> index 000000000000..4f5ecf51ed38
> --- /dev/null
> +++ b/tools/perf/util/arm64-frame-pointer-unwind-support.c
> @@ -0,0 +1,63 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include "arm64-frame-pointer-unwind-support.h"
> +#include "callchain.h"
> +#include "event.h"
> +#include "perf_regs.h" // SMPL_REG_MASK
> +#include "unwind.h"
> +
> +#define perf_event_arm_regs perf_event_arm64_regs
> +#include "../arch/arm64/include/uapi/asm/perf_regs.h"
> +#undef perf_event_arm_regs
> +
> +struct entries {
> +	u64 stack[2];
> +	size_t length;
> +};
> +
> +static bool get_leaf_frame_caller_enabled(struct perf_sample *sample)
> +{
> +	return callchain_param.record_mode == CALLCHAIN_FP && sample->user_regs.regs
> +		&& sample->user_regs.mask & SMPL_REG_MASK(PERF_REG_ARM64_LR);
> +}
> +
> +static int add_entry(struct unwind_entry *entry, void *arg)
> +{
> +	struct entries *entries = arg;
> +
> +	entries->stack[entries->length++] = entry->ip;
> +	return 0;
> +}
> +
> +u64 get_leaf_frame_caller_aarch64(struct perf_sample *sample, struct thread *thread, int usr_idx)
> +{
> +	int ret;
> +	struct entries entries = {};
> +	struct regs_dump old_regs = sample->user_regs;
> +
> +	if (!get_leaf_frame_caller_enabled(sample))
> +		return 0;
> +
> +	/*
> +	 * If PC and SP are not recorded, get the value of PC from the stack
> +	 * and set its mask. SP is not used when doing the unwinding but it
> +	 * still needs to be set to prevent failures.
> +	 */
> +
> +	if (!(sample->user_regs.mask & SMPL_REG_MASK(PERF_REG_ARM64_PC))) {
> +		sample->user_regs.cache_mask |= SMPL_REG_MASK(PERF_REG_ARM64_PC);
> +		sample->user_regs.cache_regs[PERF_REG_ARM64_PC] = sample->callchain->ips[usr_idx+1];
> +	}
> +
> +	if (!(sample->user_regs.mask & SMPL_REG_MASK(PERF_REG_ARM64_SP))) {
> +		sample->user_regs.cache_mask |= SMPL_REG_MASK(PERF_REG_ARM64_SP);
> +		sample->user_regs.cache_regs[PERF_REG_ARM64_SP] = 0;
> +	}
> +
> +	ret = unwind__get_entries(add_entry, &entries, thread, sample, 2);
> +	sample->user_regs = old_regs;
> +
> +	if (ret || entries.length != 2)
> +		return ret;
> +
> +	return callchain_param.order == ORDER_CALLER ? entries.stack[0] : entries.stack[1];
> +}
> diff --git a/tools/perf/util/arm64-frame-pointer-unwind-support.h b/tools/perf/util/arm64-frame-pointer-unwind-support.h
> new file mode 100644
> index 000000000000..32af9ce94398
> --- /dev/null
> +++ b/tools/perf/util/arm64-frame-pointer-unwind-support.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __PERF_ARM_FRAME_POINTER_UNWIND_SUPPORT_H
> +#define __PERF_ARM_FRAME_POINTER_UNWIND_SUPPORT_H
> +
> +#include "event.h"
> +#include "thread.h"
> +
> +u64 get_leaf_frame_caller_aarch64(struct perf_sample *sample, struct thread *thread, int user_idx);
> +
> +#endif /* __PERF_ARM_FRAME_POINTER_UNWIND_SUPPORT_H */
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index 3eddad009f78..a00fd6796b35 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -34,6 +34,7 @@
>  #include "bpf-event.h"
>  #include <internal/lib.h> // page_size
>  #include "cgroup.h"
> +#include "arm64-frame-pointer-unwind-support.h"
>  
>  #include <linux/ctype.h>
>  #include <symbol/kallsyms.h>
> @@ -2710,10 +2711,13 @@ static int find_prev_cpumode(struct ip_callchain *chain, struct thread *thread,
>  	return err;
>  }
>  
> -static u64 get_leaf_frame_caller(struct perf_sample *sample __maybe_unused,
> -		struct thread *thread __maybe_unused, int usr_idx __maybe_unused)
> +static u64 get_leaf_frame_caller(struct perf_sample *sample,
> +		struct thread *thread, int usr_idx)
>  {
> -	return 0;
> +	if (machine__normalize_is(thread->maps->machine, "arm64"))
> +		return get_leaf_frame_caller_aarch64(sample, thread, usr_idx);
> +	else
> +		return 0;
>  }
>  
>  static int thread__resolve_callchain_sample(struct thread *thread,
> @@ -3114,14 +3118,19 @@ int machine__set_current_tid(struct machine *machine, int cpu, pid_t pid,
>  }
>  
>  /*
> - * Compares the raw arch string. N.B. see instead perf_env__arch() if a
> - * normalized arch is needed.
> + * Compares the raw arch string. N.B. see instead perf_env__arch() or
> + * machine__normalize_is() if a normalized arch is needed.
>   */
>  bool machine__is(struct machine *machine, const char *arch)
>  {
>  	return machine && !strcmp(perf_env__raw_arch(machine->env), arch);
>  }
>  
> +bool machine__normalize_is(struct machine *machine, const char *arch)
> +{
> +	return machine && !strcmp(perf_env__arch(machine->env), arch);
> +}
> +

I think this function name would be clearer as something like "machine__normalized_is" or
"machine__normalized_arch_is". The tense is slightly off because it's a test rather than a
verb.

With that change, for the whole set:

Reviewed-by: James Clark <james.clark@xxxxxxx>


>  int machine__nr_cpus_avail(struct machine *machine)
>  {
>  	return machine ? perf_env__nr_cpus_avail(machine->env) : 0;
> diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
> index a143087eeb47..665535153411 100644
> --- a/tools/perf/util/machine.h
> +++ b/tools/perf/util/machine.h
> @@ -208,6 +208,7 @@ static inline bool machine__is_host(struct machine *machine)
>  }
>  
>  bool machine__is(struct machine *machine, const char *arch);
> +bool machine__normalize_is(struct machine *machine, const char *arch);
>  int machine__nr_cpus_avail(struct machine *machine);
>  
>  struct thread *__machine__findnew_thread(struct machine *machine, pid_t pid, pid_t tid);
>