Re: [PATCH v4 46/48] mm: shrinker: make memcg slab shrink lockless

Qi Zheng
2023-08-08T19:17:35Z

Hi Dave,

On 2023/8/8 10:44, Dave Chinner wrote:
> On Mon, Aug 07, 2023 at 07:09:34PM +0800, Qi Zheng wrote:
>> Like global slab shrink, this commit also uses refcount+RCU method to make
>> memcg slab shrink lockless.
>
> This patch does random code cleanups amongst the actual RCU changes.
> Can you please move the cleanups to a spearate patch to reduce the
> noise in this one?

Sure, will do.

>
>> diff --git a/mm/shrinker.c b/mm/shrinker.c
>> index d318f5621862..fee6f62904fb 100644
>> --- a/mm/shrinker.c
>> +++ b/mm/shrinker.c
>> @@ -107,6 +107,12 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
>> lockdep_is_held(&shrinker_rwsem));
>> }
>>
>> +static struct shrinker_info *shrinker_info_rcu(struct mem_cgroup *memcg,
>> + int nid)
>> +{
>> + return rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
>> +}
>
> This helper doesn't add value. It doesn't tell me that
> rcu_read_lock() needs to be held when it is called, for one....

How about adding a comment or an assertion here?

>
>> static int expand_one_shrinker_info(struct mem_cgroup *memcg, int new_size,
>> int old_size, int new_nr_max)
>> {
>> @@ -198,7 +204,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
>> struct shrinker_info_unit *unit;
>>
>> rcu_read_lock();
>> - info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
>> + info = shrinker_info_rcu(memcg, nid);
>
> ... whilst the original code here was obviously correct.
>
>> unit = info->unit[shriner_id_to_index(shrinker_id)];
>> if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
>> /* Pairs with smp mb in shrink_slab() */
>> @@ -211,7 +217,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
>>
>> static DEFINE_IDR(shrinker_idr);
>>
>> -static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>> +static int shrinker_memcg_alloc(struct shrinker *shrinker)
>
> Cleanups in a separate patch.

OK.

>
>> @@ -253,10 +258,15 @@ static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker,
>> {
>> struct shrinker_info *info;
>> struct shrinker_info_unit *unit;
>> + long nr_deferred;
>>
>> - info = shrinker_info_protected(memcg, nid);
>> + rcu_read_lock();
>> + info = shrinker_info_rcu(memcg, nid);
>> unit = info->unit[shriner_id_to_index(shrinker->id)];
>> - return atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0);
>> + nr_deferred = atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0);
>> + rcu_read_unlock();
>> +
>> + return nr_deferred;
>> }
>
> This adds two rcu_read_lock() sections to every call to
> do_shrink_slab(). It's not at all clear ifrom any of the other code
> that do_shrink_slab() now has internal rcu_read_lock() sections....

The xchg_nr_deferred_memcg() will only be called in shrink_slab_memcg(),
so other code doesn't need to know that information?

>
>> @@ -464,18 +480,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>> if (!mem_cgroup_online(memcg))
>> return 0;
>>
>> - if (!down_read_trylock(&shrinker_rwsem))
>> - return 0;
>> -
>> - info = shrinker_info_protected(memcg, nid);
>> +again:
>> + rcu_read_lock();
>> + info = shrinker_info_rcu(memcg, nid);
>> if (unlikely(!info))
>> goto unlock;
>>
>> - for (; index < shriner_id_to_index(info->map_nr_max); index++) {
>> + if (index < shriner_id_to_index(info->map_nr_max)) {
>> struct shrinker_info_unit *unit;
>>
>> unit = info->unit[index];
>>
>> + /*
>> + * The shrinker_info_unit will not be freed, so we can
>> + * safely release the RCU lock here.
>> + */
>> + rcu_read_unlock();
>
> Why - what guarantees that the shrinker_info_unit exists at this
> point? We hold no reference to it, we hold no reference to any
> shrinker, etc. What provides this existence guarantee?

The shrinker_info_unit is never freed unless the memcg is destroyed.
Here we hold the refcount of this memcg (mem_cgroup_iter() -->
css_tryget()), so the shrinker_info_unit will not be freed.

>
>> +
>> for_each_set_bit(offset, unit->map, SHRINKER_UNIT_BITS) {
>> struct shrink_control sc = {
>> .gfp_mask = gfp_mask,
>> @@ -485,12 +506,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>> struct shrinker *shrinker;
>> int shrinker_id = calc_shrinker_id(index, offset);
>>
>> + rcu_read_lock();
>> shrinker = idr_find(&shrinker_idr, shrinker_id);
>> - if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) {
>> - if (!shrinker)
>> - clear_bit(offset, unit->map);
>> + if (unlikely(!shrinker || !shrinker_try_get(shrinker))) {
>> + clear_bit(offset, unit->map);
>> + rcu_read_unlock();
>> continue;
>> }
>> + rcu_read_unlock();
>>
>> /* Call non-slab shrinkers even though kmem is disabled */
>> if (!memcg_kmem_online() &&
>> @@ -523,15 +546,20 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>> set_shrinker_bit(memcg, nid, shrinker_id);
>> }
>> freed += ret;
>> -
>> - if (rwsem_is_contended(&shrinker_rwsem)) {
>> - freed = freed ? : 1;
>> - goto unlock;
>> - }
>> + shrinker_put(shrinker);
>
> Ok, so why is this safe to call without holding the rcu read lock?
> The global shrinker has to hold the rcu_read_lock() whilst calling
> shrinker_put() to guarantee the validity of the list next pointer,
> but we don't hold off RCU here so what guarantees a racing global
> shrinker walk doesn't trip over this shrinker_put() call dropping
> the refcount to zero and freeing occuring in a different context...

This will not be a problem, even if shrinker::refcount is reduced to
0 here, the racing global shrinker walk already holds the rcu lock.

shrink_slab shrink_slab_memcg
=========== =================

rcu_read_lock()
shrinker_put()
shrinker_put()

And in shrink_slab_memcg(), the shrinker is not required to traverse the
next bit in the shrinker_info_unit::map, so there is no need to hold the
rcu lock to ensure the existence of this shrinker.

>
>
>> + /*
>> + * We have already exited the read-side of rcu critical section
>> + * before calling do_shrink_slab(), the shrinker_info may be
>> + * released in expand_one_shrinker_info(), so reacquire the
>> + * shrinker_info.
>> + */
>> + index++;
>> + goto again;
>
> With that, what makes the use of shrinker_info in
> xchg_nr_deferred_memcg() in do_shrink_slab() coherent and valid?

Holding rcu lock can ensure that the old shrinker_info will not be
freed, and the shrinker_info_unit::nr_deferred can also be indexed from
the old shrinker_info::unit[x], so the updated nr_deferred will not be
lost.

Thanks,
Qi

>
> -Dave.