for_each_possible_cpu macro in vmalloc_init() function, does the code run in only one cpu? or in every cpu?

Question

this is not about programming, but I ask it here.. in linux start_kernel() function, in the mm_init() function, I see vmalloc_init() function. inside the function I see codes like this.

void __init vmalloc_init(void)
{
    struct vmap_area *va;
    struct vm_struct *tmp;
    int i;
        
    /*  
     * Create the cache for vmap_area objects.
     */
    vmap_area_cachep = KMEM_CACHE(vmap_area, SLAB_PANIC);

    for_each_possible_cpu(i) {
        struct vmap_block_queue *vbq;
        struct vfree_deferred *p;
                    
        vbq = &per_cpu(vmap_block_queue, i);
        spin_lock_init(&vbq->lock);
        INIT_LIST_HEAD(&vbq->free);
        p = &per_cpu(vfree_deferred, i);
        init_llist_head(&p->list);
        INIT_WORK(&p->wq, free_work);
    }

    /* Import existing vmlist entries. */
    for (tmp = vmlist; tmp; tmp = tmp->next) {
        va = kmem_cache_zalloc(vmap_area_cachep, GFP_NOWAIT);
        if (WARN_ON_ONCE(!va))
            continue;

        va->va_start = (unsigned long)tmp->addr;
        va->va_end = va->va_start + tmp->size;
        va->vm = tmp;
        insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
    }

    /*
     * Now we can initialize a free vmap space.
     */
    vmap_init_free_space();
    vmap_initialized = true;
}

I'm not sure if this code is run on every cpu(core) or just on the first cpu?
if this code runs on every smp core, how is this code inside for_each_possible_cpu loop run?
The smp setup seems to be done before this function.

score 2 · Accepted Answer · answered Aug 25 '20 at 13:51

2

start_kernel() calls mm_init() which calls vmalloc_init(). Only the first (boot) CPU is active at that point. Later, start_kernel() calls arch_call_rest_init() which calls rest_init().

rest_init() creates a kernel thread for the init task with entry point kernel_init(). kernel_init() calls kernel_init_freeable(). kernel_init_freeable() eventually calls smp_init() to activate the remaining CPUs.

answered Aug 25 '20 at 13:51

Ian Abbott

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ah that was so. Thank you! – Chan Kim Aug 26 '20 at 08:53

score 1 · Answer 2 · answered Aug 25 '20 at 19:34

Every macro in for_each_cpu family is just wrapper for for() loop, where iterator is a CPU index.

E.g., the core macro of this family is defined as

#define for_each_cpu(cpu, mask)             \
    for ((cpu) = -1;                \
        (cpu) = cpumask_next((cpu), (mask)),    \
        (cpu) < nr_cpu_ids;)

Each macro in for_each_cpu family uses its own CPUs mask, which is just a set of bits corresponded to CPU indices. E.g. mask for for_each_possible_cpu macro have bits set for every index of CPU which could ever be enabled in current machine session.

for_each_possible_cpu macro in vmalloc_init() function, does the code run in only one cpu? or in every cpu?

2 Answers2

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