[Axstd] Dropping thread::JoinHandle doesn't run the task

[zjp-CN]

Axstd mocks libstd APIs, and JoinHandle is documented as follows:

arceos/ulib/axstd/src/thread/multi.rs

Lines 158 to 165 in 9b740db

	/// A `JoinHandle` *detaches* the associated thread when it is dropped, which
	/// means that there is no longer any handle to the thread and no way to `join`
	/// on it.
	pub struct JoinHandle<T> {
	native: AxTaskHandle,
	thread: Thread,
	packet: Arc<Packet<T>>,
	}

It means when JoinHandle is dropped, the thread is still running behind the scenes. playground

However, ArceOS doesn't schedule the thread at all. Here's a demo:

# Cargo.toml
axstd = { workspace = true, features = ["multitask"] }

#![no_std]
#![no_main]

use axstd::{println, thread};
use core::sync::atomic::{AtomicU64, Ordering};

#[unsafe(no_mangle)]
fn main() {
    let n = AtomicU64::new(0);
    loop {
        let id = n.fetch_add(1, Ordering::Relaxed);

        _ = thread::spawn(move || println!("{id}")).join(); // (1)
        // drop(thread::spawn(move || println!("{id}"))); // or (2)

        if id == 10000 {
            break;
        }
    }
    println!("main stops.");
}

(1) works well because it waits for the task to finish, and all the 10000 threads are successfully run.
(2) causes OOM panic: no task is run at all, so memory runs out.

[  0.029819 0:2 axruntime::lang_items:5] 
panicked at library/alloc/src/alloc.rs:437:13: memory allocation of 262144 bytes failed

[zjp-CN]

It seems the expected behavior, because there is 1 scheduling core available by default, the task queue is no way to proceed.

Setting SMP to a larger number can make it run.

$ SMP=2 make run
smp = 2
1
3
5
7
9
[  0.039798 0:2 axruntime::lang_items:5] panicked at library/alloc/src/alloc.rs:437:13:
memory allocation of 262144 bytes failed
11
13

The OOM still happens, but it makes sense since threads are created too fast before the rest finishes.

[zjp-CN]

But the scheduling is too slow: with SMP=4, OOM happens after 540 threads finishes.

fn main() {
    let n = AtomicU64::new(0);
    loop {
        let id = n.fetch_add(1, Ordering::Relaxed);

        drop(thread::spawn(move || println!("{id}: spawned"))); // (2)

        println!("{id} loop");
        if id == 10000 {
            break;
        }
    }
    thread::sleep(Duration::from_secs(3));
    println!("main stops.");
}

...
539 loop
539: spawned
540: spawned
540 loop
[  0.293891 1:549 axtask::task:563] 549 task_entry
[  0.293985 2:550 axtask::task:563] 550 task_entry
[  0.299331 0:2 axruntime::lang_items:5] panicked at library/alloc/src/alloc.rs:437:13:
memory allocation of 262144 bytes failed
[  0.300504 0:2 axplat_x86_pc::power:25] Shutting down...