Re: [PATCH 2/5] mm: vmscan: make memcg slab shrink lockless

[Qi Zheng]

On 2023/2/23 03:58, Kirill Tkhai wrote:
> On 22.02.2023 10:32, Qi Zheng wrote:
> > On 2023/2/22 05:28, Kirill Tkhai wrote:
> > > On 20.02.2023 12:16, Qi Zheng wrote:
> > <...>
> > > >      void reparent_shrinker_deferred(struct mem_cgroup *memcg)
> > > >    {
> > > > -    int i, nid;
> > > > +    int i, nid, srcu_idx;
> > > >        long nr;
> > > >        struct mem_cgroup *parent;
> > > >        struct shrinker_info *child_info, *parent_info;
> > > > @@ -429,16 +443,16 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
> > > >            parent = root_mem_cgroup;
> > > >          /* Prevent from concurrent shrinker_info expand */
> > > > -    down_read(&shrinker_rwsem);
> > > > +    srcu_idx = srcu_read_lock(&shrinker_srcu);
> > >
> > > Don't we still have to be protected against parallel expand_one_shrinker_info()?
> > >
> > > It looks like parent->nodeinfo[nid]->shrinker_info pointer may be changed in expand*
> > > right after we've dereferenced it here.
> >
> > Hi Kirill,
> >
> > Oh, indeed. We may wrongly reparent the child's nr_deferred to the old
> > parent's nr_deferred (it is about to be freed by call_srcu).
> >
> > The reparent_shrinker_deferred() will only be called on the offline
> > path (not a hotspot path), so we may be able to use shrinker_mutex
> > introduced later for protection. What do you think?
>
> It looks good for me. One more thing I'd analyzed is whether we want to have
> is two reparent_shrinker_deferred() are executing in parallel.

I see that mem_cgroup_css_offline() is already protected by
cgroup_mutex, so maybe shrinker_mutex is enough here. :)

> Possible, we should leave rwsem there as it was used before..
>   
> > > >        for_each_node(nid) {
> > > > -        child_info = shrinker_info_protected(memcg, nid);
> > > > -        parent_info = shrinker_info_protected(parent, nid);
> > > > +        child_info = shrinker_info_srcu(memcg, nid);
> > > > +        parent_info = shrinker_info_srcu(parent, nid);
> > > >            for (i = 0; i < shrinker_nr_max; i++) {
> > > >                nr = atomic_long_read(&child_info->nr_deferred[i]);
> > > >                atomic_long_add(nr, &parent_info->nr_deferred[i]);
> > > >            }
> > > >        }
> > > > -    up_read(&shrinker_rwsem);
> > > > +    srcu_read_unlock(&shrinker_srcu, srcu_idx);
> > > >    }
> > > >      static bool cgroup_reclaim(struct scan_control *sc)
> > > > @@ -891,15 +905,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> > > >    {
> > > >        struct shrinker_info *info;
> > > >        unsigned long ret, freed = 0;
> > > > +    int srcu_idx;
> > > >        int i;
> > > >          if (!mem_cgroup_online(memcg))
> > > >            return 0;
> > > >    -    if (!down_read_trylock(&shrinker_rwsem))
> > > > -        return 0;
> > > > -
> > > > -    info = shrinker_info_protected(memcg, nid);
> > > > +    srcu_idx = srcu_read_lock(&shrinker_srcu);
> > > > +    info = shrinker_info_srcu(memcg, nid);
> > > >        if (unlikely(!info))
> > > >            goto unlock;
> > > >    @@ -949,14 +962,9 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> > > >                    set_shrinker_bit(memcg, nid, i);
> > > >            }
> > > >            freed += ret;
> > > > -
> > > > -        if (rwsem_is_contended(&shrinker_rwsem)) {
> > > > -            freed = freed ? : 1;
> > > > -            break;
> > > > -        }
> > > >        }
> > > >    unlock:
> > > > -    up_read(&shrinker_rwsem);
> > > > +    srcu_read_unlock(&shrinker_srcu, srcu_idx);
> > > >        return freed;
> > > >    }
> > > >    #else /* CONFIG_MEMCG */

--
Thanks,
Qi