Stacktrace from other threads are not available in flamegraph, but is available from `perf report`

[oxalica]

I encountered this issue from tokio::task::spawn_blocking which execute the closure in a thread pool. But it seems that simply creating a new std thread can reproduce it.

fn main() {
    std::thread::spawn(|| {
        busy_func();
    })
    .join()
    .unwrap()
}

fn busy_func() {
    for i in 0..u32::MAX {
        std::hint::black_box(i);
    }
}

I'm expecting busy_func being shown as the main time cost, but cargo flamegraph shows nothing relevant and misleading says _start costs most of the time.

Using the captured perf.data, running perf report shows the hot function correctly:

Samples: 984  of event 'cpu/cycles/Pu', Event count (approx.): 4306731901
  Children      Self  Command  Shared Object         Symbol
   100.00%   100.00%  main     main                  [.] std::sys::backtrace::__rust_begin_short_backtrace::<main::main::{closure#0}, ()>
     0.00%     0.00%  main     ld-linux-x86-64.so.2  [.] _start
     0.00%     0.00%  main     ld-linux-x86-64.so.2  [.] _dl_start
     0.00%     0.00%  main     ld-linux-x86-64.so.2  [.] _dl_lookup_symbol_x
     0.00%     0.00%  main     ld-linux-x86-64.so.2  [.] _dl_sysdep_start
     0.00%     0.00%  main     ld-linux-x86-64.so.2  [.] dl_main

The first function is the thread entry point with busy_func inlined. Clicking into it shows:

Samples: 984  of event 'cpu/cycles/Pu', 997 Hz, Event count (approx.): 4306731901
std::sys::backtrace::__rust_begin_short_backtrace::<main::main::{closure#0}, ()>  [redacted]/target/release/main [Percent: local period]
Percent │      xor  %eax,%eax
        │      lea  -0x4(%rsp),%rcx
        │      nop
        │10:   mov  %eax,-0x4(%rsp)
  73.76 │      inc  %eax
        │      cmp  $0xffffffff,%eax
  26.24 │    ↑ jne  10
        │    ← ret

More informations

Linux 6.18.24
perf 7.0.1
rustc 1.95.0 (59807616e 2026-04-14)
flamegraph-flamegraph 0.6.12

I'm using LLD and already have -Clink-arg=-Wl,--no-rosegment argument set.

[djc]

I won't have time to look into this -- happy to review a PR if you're able to make some progress.