Re: [PATCH v1] perf machine: Don't unwind the arm64 leaf frame caller without symbols

[Ian Rogers]

On Wed, Oct 14, 2026 at 8:13 AM Matthew Wood <thepacketgeek@xxxxxxxxx> wrote:
>
> On arm64, 'perf record --call-graph fp' also samples LR so that
> thread__resolve_callchain_sample() can recover the leaf function's
> caller, which is missing from the frame pointer chain. It does so via
> get_leaf_frame_caller_aarch64(), which runs a DWARF unwind. The
> unwinder looks up the symbol of every frame it visits, so the first
> lookup in a DSO loads its full symbol table and demangles every symbol
> in it.
>
> Commit 02b2705017d2 ("perf callchain: Allow symbols to be optional when
> resolving a callchain") added the symbols argument so that callers which
> only need maps can avoid loading symbols, and passed it down to every
> add_callchain_ip() call. The leaf frame unwind was missed and still runs
> for symbols=false, so 'perf inject' on arm64 has been loading symbols it
> doesn't need ever since.
>
> Commit 29be60c93d2d ("perf build-id: Mark DSO in sample callchains")
> compounded the issue: process_buildids() now walks the callchain of
> every sample at the end of 'perf record' to mark DSOs hit. With -a, that
> loads and demangles the symbol tables of every user binary sampled on
> the system. This can send things into exponential time and memory
> (over 1GB of heap growth in 5 seconds), so
>
>   perf record -a -e cycles:u --call-graph fp -- sleep 2
>
> never finishes writing perf.data after the workload exits.
>
> Callers that pass symbols=false only want the maps that callchain IPs
> fall in, and add_callchain_ip() already honours that by using
> thread__find_map() instead of thread__find_symbol(). Skip the leaf
> frame unwind for them as well. The leaf frame caller is almost always
> in the same DSO as the sampled IP or the next frame in the chain, so
> build-id marking loses nothing in practice.
>
> Fixes: 02b2705017d2 ("perf callchain: Allow symbols to be optional when resolving a callchain")
> Signed-off-by: Matthew Wood <thepacketgeek@xxxxxxxxx>
> ---
>  tools/perf/util/machine.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index f86b3b7df742..51597cddd239 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -2989,7 +2989,13 @@ static int thread__resolve_callchain_sample(struct thread *thread,
>
>                 usr_idx = callchain_param.order == ORDER_CALLEE ? j-2 : j-1;
>
> -               if (usr_idx >= 0 && chain->ips[usr_idx] == PERF_CONTEXT_USER) {
> +               /*
> +                * Finding the leaf frame caller needs a DWARF unwind, and the
> +                * unwinder looks up the symbol of every frame it visits. Skip
> +                * it when the caller asked for maps only, as for build-id
> +                * marking, so that no symbol tables are loaded.
> +                */
> +               if (symbols && usr_idx >= 0 && chain->ips[usr_idx] == PERF_CONTEXT_USER) {

Hi Matthew, I'm not sure if this is a serious patch or a bug report
about the slowness of DSO marking. Broadly DSO marking is known to be
slow and many perf tests run with -B and -N for this reason. I believe
the intent behind DSO marking as a default is to ensure that perf
provides the best profiling experience. This comes at a cost, but this
fix looks like it will break things because IPs will be lost.

Trying to look a little more at the ARM64 performance problem, I see
that we're trying to unwind 2 frames:
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/arm64-frame-pointer-unwind-support.c?h=perf-tools-next#n70
but often just 1 is read. For libunwind the 1 frame case has a special case:
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/unwind-libunwind.c?h=perf-tools-next#n671
I wonder if we could replicate the 1 frame case with libdw to avoid
processing the DSO, and update the caller to be:
```
ret = unwind__get_entries(add_entry, &entries, thread, sample,
callchain_param.order == ORDER_CALLER ? 1 : 2, true);
```

Thanks,
Ian

>
>                         leaf_frame_caller = get_leaf_frame_caller(sample, thread, usr_idx);
>
> --
> 2.53.0-Meta