`_remote_debugging`: no need to update `last_profiled_frame` if no change

[maurycy]

Bug report

Bug description:

I'm investigating if profiling.sampling can be faster, and noticed a massive number of process_vm_writev syscalls:

2026-05-07T23:54:53.689606110+0000 maurycy@weiss /home/maurycy/cpython (main f5c7535*) # ./python.exe /tmp/spin.py & TGT=$!
sleep 0.5
strace -c -f -e trace=process_vm_writev,process_vm_readv,pwritev,pwritev2 \
  ./python.exe -m profiling.sampling attach -r 100khz -d 30 \
  --pstats -o /tmp/out.pstats $TGT
kill $TGT; wait 2>/dev/null
[1] 135687
Captured 118240 samples in 30.00 seconds
Sample rate: 3941.33 samples/sec
Error rate: 0.00
Warning: missed 2881765 samples from the expected total of 3000005 (96.06%)
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ------------------
 75.87    0.916719           2    354732           process_vm_readv
 24.13    0.291560           2    118241           process_vm_writev
------ ----------- ----------- --------- --------- ------------------
100.00    1.208279           2    472973           total
[1]  + terminated  ./python.exe /tmp/spin.py
2026-05-07T23:58:28.503095895+0000 maurycy@weiss /home/maurycy/cpython (remote-debugging-last-profiled-frame 1a07c4a*) # cat /tmp/spin.py 
def f():
    x = 0
    while True:
        x += 1
f()

(This is a Docker instance, and I'm too tired to configure /sys/kernel/tracing properly, so using strace instead of perf)

Given how the script looks, the last frame should be literally the same.

I believe this is the reason:

cpython/Modules/_remote_debugging/threads.c

Lines 453 to 459 in b142878

	// Update last_profiled_frame for next sample
	uintptr_t lpf_addr =
	*current_tstate + (uintptr_t)unwinder->debug_offsets.thread_state.last_profiled_frame;
	if (_Py_RemoteDebug_WriteRemoteMemory(&unwinder->handle, lpf_addr,
	sizeof(uintptr_t), &frame_addr) < 0) {
	PyErr_Clear(); // Non-fatal
	}

The last_profiled_frame is updated no matter what, and _Py_RemoteDebug_WriteRemoteMemory results in a syscall:

cpython/Python/remote_debug.h

Line 1234 in 49918f5

written = process_vm_writev(handle->pid, local, 1, remote, 1, 0);

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-149521: Do not update last_profiled_frame if it's not changed #149522