Re: [PATCH v6 42/45] mm: shrinker: make global slab shrink lockless

Lai Jiangshan
2023-12-06T07:47:58Z

On Tue, Sep 12, 2023 at 9:57 PM Qi Zheng <zhengqi.arch@bytedance.com> wrote:

> - if (!down_read_trylock(&shrinker_rwsem))
> - goto out;
> -
> - list_for_each_entry(shrinker, &shrinker_list, list) {
> + /*
> + * lockless algorithm of global shrink.
> + *
> + * In the unregistration setp, the shrinker will be freed asynchronously
> + * via RCU after its refcount reaches 0. So both rcu_read_lock() and
> + * shrinker_try_get() can be used to ensure the existence of the shrinker.
> + *
> + * So in the global shrink:
> + * step 1: use rcu_read_lock() to guarantee existence of the shrinker
> + * and the validity of the shrinker_list walk.
> + * step 2: use shrinker_try_get() to try get the refcount, if successful,
> + * then the existence of the shrinker can also be guaranteed,
> + * so we can release the RCU lock to do do_shrink_slab() that
> + * may sleep.
> + * step 3: *MUST* to reacquire the RCU lock before calling shrinker_put(),
> + * which ensures that neither this shrinker nor the next shrinker
> + * will be freed in the next traversal operation.

Hello, Qi, Andrew, Paul,

I wonder know how RCU can ensure the lifespan of the next shrinker.
it seems it is diverged from the common pattern usage of RCU+reference.

cpu1:
rcu_read_lock();
shrinker_try_get(this_shrinker);
rcu_read_unlock();
cpu2: shrinker_free(this_shrinker);
cpu2: shrinker_free(next_shrinker); and free the memory of next_shrinker
cpu2: when shrinker_free(next_shrinker), no one updates this_shrinker's next
cpu2: since this_shrinker has been removed first.
rcu_read_lock();
shrinker_put(this_shrinker);
travel to the freed next_shrinker.

a quick simple fix:

// called with other references other than RCU (i.e. refcount)
static inline rcu_list_deleted(struct list_head *entry)
{
// something like this:
return entry->prev == LIST_POISON2;
}

// in the loop
if (rcu_list_deleted(&shrinker->list)) {
shrinker_put(shrinker);
goto restart;
}
rcu_read_lock();
shrinker_put(shrinker);

Thanks
Lai

> + * step 4: do shrinker_put() paired with step 2 to put the refcount,
> + * if the refcount reaches 0, then wake up the waiter in
> + * shrinker_free() by calling complete().
> + */
> + rcu_read_lock();
> + list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
> struct shrink_control sc = {
> .gfp_mask = gfp_mask,
> .nid = nid,
> .memcg = memcg,
> };
>
> + if (!shrinker_try_get(shrinker))
> + continue;
> +
> + rcu_read_unlock();
> +
> ret = do_shrink_slab(&sc, shrinker, priority);
> if (ret == SHRINK_EMPTY)
> ret = 0;
> freed += ret;
> - /*
> - * Bail out if someone want to register a new shrinker to
> - * prevent the registration from being stalled for long periods
> - * by parallel ongoing shrinking.
> - */
> - if (rwsem_is_contended(&shrinker_rwsem)) {
> - freed = freed ? : 1;
> - break;
> - }
> +
> + rcu_read_lock();
> + shrinker_put(shrinker);
> }
>