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

[Qi Zheng]

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.