Re: [PATCH] perf: Don't show unwind error messages when augmenting frame pointer stack

[John Garry]

On 06/04/2022 15:56, James Clark wrote:
> Commit b9f6fbb3b2c2 ("perf arm64: Inject missing frames when using 'perf
> record --call-graph=fp'") intended to add a 'best effort' Dwarf unwind
> that improved the frame pointer stack in most scenarios. It's expected
> that the unwind will fail sometimes, but this shouldn't be reported as
> an error. It only works when the return address can be determined from
> the contents of the link register alone.
>
> Fix the error shown when the unwinder requires extra registers by adding
> a new flag that suppresses error messages. This flag is not set in the
> normal --call-graph=dwarf unwind mode so that behavior is not changed.
>
> Reported-by: John Garry <john.garry@xxxxxxxxxx>
> Fixes: b9f6fbb3b2c2 ("perf arm64: Inject missing frames when using 'perf record --call-graph=fp'")
> Signed-off-by: James Clark <james.clark@xxxxxxx>

Seems ok:
Tested-by: John Garry <john.garry@xxxxxxxxxx>

Thanks!

> ---
>   tools/perf/tests/dwarf-unwind.c                     |  2 +-
>   .../perf/util/arm64-frame-pointer-unwind-support.c  |  2 +-
>   tools/perf/util/machine.c                           |  2 +-
>   tools/perf/util/unwind-libdw.c                      | 10 +++++++---
>   tools/perf/util/unwind-libdw.h                      |  1 +
>   tools/perf/util/unwind-libunwind-local.c            | 10 +++++++---
>   tools/perf/util/unwind-libunwind.c                  |  6 ++++--
>   tools/perf/util/unwind.h                            | 13 ++++++++++---
>   8 files changed, 32 insertions(+), 14 deletions(-)
>
> diff --git a/tools/perf/tests/dwarf-unwind.c b/tools/perf/tests/dwarf-unwind.c
> index 2dab2d262060..afdca7f2959f 100644
> --- a/tools/perf/tests/dwarf-unwind.c
> +++ b/tools/perf/tests/dwarf-unwind.c
> @@ -122,7 +122,7 @@ NO_TAIL_CALL_ATTRIBUTE noinline int test_dwarf_unwind__thread(struct thread *thr
>   	}
>   
>   	err = unwind__get_entries(unwind_entry, &cnt, thread,
> -				  &sample, MAX_STACK);
> +				  &sample, MAX_STACK, false);
>   	if (err)
>   		pr_debug("unwind failed\n");
>   	else if (cnt != MAX_STACK) {
> diff --git a/tools/perf/util/arm64-frame-pointer-unwind-support.c b/tools/perf/util/arm64-frame-pointer-unwind-support.c
> index 2242a885fbd7..4940be4a0569 100644
> --- a/tools/perf/util/arm64-frame-pointer-unwind-support.c
> +++ b/tools/perf/util/arm64-frame-pointer-unwind-support.c
> @@ -53,7 +53,7 @@ u64 get_leaf_frame_caller_aarch64(struct perf_sample *sample, struct thread *thr
>   		sample->user_regs.cache_regs[PERF_REG_ARM64_SP] = 0;
>   	}
>   
> -	ret = unwind__get_entries(add_entry, &entries, thread, sample, 2);
> +	ret = unwind__get_entries(add_entry, &entries, thread, sample, 2, true);
>   	sample->user_regs = old_regs;
>   
>   	if (ret || entries.length != 2)
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index b80048546451..95391236f5f6 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -2987,7 +2987,7 @@ static int thread__resolve_callchain_unwind(struct thread *thread,
>   		return 0;
>   
>   	return unwind__get_entries(unwind_entry, cursor,
> -				   thread, sample, max_stack);
> +				   thread, sample, max_stack, false);
>   }
>   
>   int thread__resolve_callchain(struct thread *thread,
> diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c
> index a74b517f7497..94aa40f6e348 100644
> --- a/tools/perf/util/unwind-libdw.c
> +++ b/tools/perf/util/unwind-libdw.c
> @@ -200,7 +200,8 @@ frame_callback(Dwfl_Frame *state, void *arg)
>   	bool isactivation;
>   
>   	if (!dwfl_frame_pc(state, &pc, NULL)) {
> -		pr_err("%s", dwfl_errmsg(-1));
> +		if (!ui->best_effort)
> +			pr_err("%s", dwfl_errmsg(-1));
>   		return DWARF_CB_ABORT;
>   	}
>   
> @@ -208,7 +209,8 @@ frame_callback(Dwfl_Frame *state, void *arg)
>   	report_module(pc, ui);
>   
>   	if (!dwfl_frame_pc(state, &pc, &isactivation)) {
> -		pr_err("%s", dwfl_errmsg(-1));
> +		if (!ui->best_effort)
> +			pr_err("%s", dwfl_errmsg(-1));
>   		return DWARF_CB_ABORT;
>   	}
>   
> @@ -222,7 +224,8 @@ frame_callback(Dwfl_Frame *state, void *arg)
>   int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
>   			struct thread *thread,
>   			struct perf_sample *data,
> -			int max_stack)
> +			int max_stack,
> +			bool best_effort)
>   {
>   	struct unwind_info *ui, ui_buf = {
>   		.sample		= data,
> @@ -231,6 +234,7 @@ int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
>   		.cb		= cb,
>   		.arg		= arg,
>   		.max_stack	= max_stack,
> +		.best_effort    = best_effort
>   	};
>   	Dwarf_Word ip;
>   	int err = -EINVAL, i;
> diff --git a/tools/perf/util/unwind-libdw.h b/tools/perf/util/unwind-libdw.h
> index 0cbd2650e280..8c88bc4f2304 100644
> --- a/tools/perf/util/unwind-libdw.h
> +++ b/tools/perf/util/unwind-libdw.h
> @@ -20,6 +20,7 @@ struct unwind_info {
>   	void			*arg;
>   	int			max_stack;
>   	int			idx;
> +	bool			best_effort;
>   	struct unwind_entry	entries[];
>   };
>   
> diff --git a/tools/perf/util/unwind-libunwind-local.c b/tools/perf/util/unwind-libunwind-local.c
> index 71a353349181..41e29fc7648a 100644
> --- a/tools/perf/util/unwind-libunwind-local.c
> +++ b/tools/perf/util/unwind-libunwind-local.c
> @@ -96,6 +96,7 @@ struct unwind_info {
>   	struct perf_sample	*sample;
>   	struct machine		*machine;
>   	struct thread		*thread;
> +	bool			 best_effort;