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

Lai Jiangshan
2023-12-06T08:42:54Z

On Wed, Dec 6, 2023 at 3:55 PM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>
> Hi,
>
> On 2023/12/6 15:47, Lai Jiangshan wrote:
> > 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.
>
> No, this_shrinker will not be removed from the shrinker_list until the
> last refcount is released. See below:

Oh, I miss the code here.

Thanks
Lai

>
> > rcu_read_lock();
> > shrinker_put(this_shrinker);
>
> CPU 1 CPU 2
>
> --> if (refcount_dec_and_test(&shrinker->refcount))
> complete(&shrinker->done);
>
> wait_for_completion(&shrinker->done);
> list_del_rcu(&shrinker->list);
>
> > 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);
> >> }
> >>