Zephyr: `k_yield()` exhibits different behavior with `CONFIG_SMP`

[ghost]

Using the latest master (currently 536e785b932b3b246891c8a8aa0075e387e1ed4a).

The documentation for k_yield() says:

Yield the current thread. This routine causes the current thread to yield execution to another thread of the same or higher priority. If there are no other ready threads of the same or higher priority, the routine returns immediately.

... but the SMP scheduler (on a single-processor system) does not appear to yield control to other equal-priority threads.

Sample program:

#include <zephyr.h>
#include <sys/printk.h>

#define STACK_SIZE 8192

K_THREAD_STACK_DEFINE(stack0, STACK_SIZE);
K_THREAD_STACK_DEFINE(stack1, STACK_SIZE);
K_THREAD_STACK_DEFINE(stack2, STACK_SIZE);

void thread_func(void *p1, void *p2, void *p3)
{
    int num = POINTER_TO_INT(p1);

    k_busy_wait(500000 * num);

    for (;;) {
        k_busy_wait(1000000);
        printk("(%d)\n", num);
        k_yield();
    }
}

struct k_thread thread0;

struct k_thread thread1;
struct k_thread thread2;

void thread_func0(void *p1, void *p2, void *p3)
{
    k_thread_create(&thread1, stack1, STACK_SIZE, thread_func,
        INT_TO_POINTER(1), NULL, NULL, 0, 0, 0);

    k_thread_create(&thread2, stack2, STACK_SIZE, thread_func,
        INT_TO_POINTER(2), NULL, NULL, 0, 0, 0);
}

void main(void)
{
    k_thread_create(&thread0, stack0, STACK_SIZE, thread_func0,
        NULL, NULL, NULL, 0, 0, 0);
}

Basically, two threads are created of equal priority which busy-loop and print an identifying number periodically, yielding each time through the loop. (thread_func0 exists solely in case the main thread has a low priority.) With the old scheduler (CONFIG_SMP=n), this works as expected, with the two threads interleaving --

On a qemu_x86 target (CONFIG_SMP=n obviously):

SeaBIOS (version rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org)
Booting from ROM..Optimal CONFIG_X86_MMU_PAGE_POOL_PAGES 7
***** Booting Zephyr OS build v2.0.0-rc3-721-g536e785b932b *****
(1)
(2)
(1)
(2)
(1)
(2)

etc.

On a qemu_x86_64 target (CONFIG_SMP=n):

SeaBIOS (version rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org)
Booting from ROM..(1)
(2)
(1)
(2)

etc.

On a qemu_x86_64 target (CONFIG_SMP=y CONFIG_MP_NUM_CPUS=1):

SeaBIOS (version rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org)
Booting from ROM..(1)
(1)
(1)
(1)
(1)

The second thread is never scheduled.

[andyross]

Did you get a chance to try over #19381?

[ghost]

Did you get a chance to try over #19381?

I clicked this link until my stack overflowed

[ghost]

a5ad00ca82cc6ca4a9636a051399b62c09fc5901 in https://github.com/zephyrproject-rtos/zephyr/pull/18531 fixes this

[andyross]

18531 (pasted the correct link this time) is merged now, we can close this.