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kernel: O(1) search for threads among CPUs #101372

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35 changes: 19 additions & 16 deletions kernel/sched.c
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Original file line number Diff line number Diff line change
Expand Up @@ -300,23 +300,24 @@ static ALWAYS_INLINE void update_cache(int preempt_ok)
#endif /* CONFIG_SMP */
}

/**
* Returns pointer to _cpu if the thread is currently running on
* another CPU.
*/
static struct _cpu *thread_active_elsewhere(struct k_thread *thread)
{
/* Returns pointer to _cpu if the thread is currently running on
* another CPU. There are more scalable designs to answer this
* question in constant time, but this is fine for now.
*/
#ifdef CONFIG_SMP
int currcpu = _current_cpu->id;
int thread_cpu_id = thread->base.cpu;
struct _cpu *thread_cpu;

unsigned int num_cpus = arch_num_cpus();
__ASSERT_NO_MSG((thread_cpu_id >= 0) &&
(thread_cpu_id < arch_num_cpus()));

for (int i = 0; i < num_cpus; i++) {
if ((i != currcpu) &&
(_kernel.cpus[i].current == thread)) {
return &_kernel.cpus[i];
}
thread_cpu = &_kernel.cpus[thread_cpu_id];
if ((thread_cpu->current == thread) && (thread_cpu != _current_cpu)) {
return thread_cpu;
}

#endif /* CONFIG_SMP */
ARG_UNUSED(thread);
return NULL;
Expand Down Expand Up @@ -386,11 +387,13 @@ static void thread_halt_spin(struct k_thread *thread, k_spinlock_key_t key)
static ALWAYS_INLINE void z_metairq_preempted_clear(struct k_thread *thread)
{
#if (CONFIG_NUM_METAIRQ_PRIORITIES > 0)
for (unsigned int i = 0; i < CONFIG_MP_MAX_NUM_CPUS; i++) {
if (_kernel.cpus[i].metairq_preempted == thread) {
_kernel.cpus[i].metairq_preempted = NULL;
break;
}
unsigned int cpu_id = 0;

#if defined(CONFIG_SMP) && (CONFIG_MP_MAX_NUM_CPUS > 1)
cpu_id = thread->base.cpu;
#endif
if (_kernel.cpus[cpu_id].metairq_preempted == thread) {
_kernel.cpus[cpu_id].metairq_preempted = NULL;
}
#endif
}
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1 change: 1 addition & 0 deletions kernel/thread.c
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Original file line number Diff line number Diff line change
Expand Up @@ -890,6 +890,7 @@ void z_init_thread_base(struct _thread_base *thread_base, int priority,

#ifdef CONFIG_SMP
thread_base->is_idle = 0;
thread_base->cpu = 0;
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@TaiJuWu TaiJuWu Dec 20, 2025

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It means the run on cpu 0 last time, should we use a magic number to indicate edge case for thread does not run before?

Additional, there is comment from _thread_base, the semantic is changed now.

/* CPU index on which thread was last run */
uint8_t cpu;

By the way, I think it is used to cpu affinity before but I am not sure we have this feature or not.

#endif /* CONFIG_SMP */

#ifdef CONFIG_TIMESLICE_PER_THREAD
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