Re: [PATCH v2 1/7] mm: vmscan: add a map_nr_max field to shrinker_info

[Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>]

On 2023/2/25 23:14, Kirill Tkhai wrote:
> Hi Qi,
>
> On 25.02.2023 11:18, Qi Zheng wrote:
> > On 2023/2/23 21:27, Qi Zheng wrote:
> > > To prepare for the subsequent lockless memcg slab shrink,
> > > add a map_nr_max field to struct shrinker_info to records
> > > its own real shrinker_nr_max.
> > >
> > > No functional changes.
> > >
> > > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> >
> > I missed Suggested-by here, hi Kirill, can I add it?
> >
> > Suggested-by: Kirill Tkhai <tkhai@xxxxx>
>
> Yes, feel free to add this tag.
>
> There is a comment below.
>
> > > ---
> > >    include/linux/memcontrol.h |  1 +
> > >    mm/vmscan.c                | 29 ++++++++++++++++++-----------
> > >    2 files changed, 19 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index b6eda2ab205d..aa69ea98e2d8 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -97,6 +97,7 @@ struct shrinker_info {
> > >        struct rcu_head rcu;
> > >        atomic_long_t *nr_deferred;
> > >        unsigned long *map;
> > > +    int map_nr_max;
> > >    };
> > >      struct lruvec_stats_percpu {
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 9c1c5e8b24b8..9f895ca6216c 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -224,9 +224,16 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
> > >                         lockdep_is_held(&shrinker_rwsem));
> > >    }
> > >    +static inline bool need_expand(int new_nr_max, int old_nr_max)
> > > +{
> > > +    return round_up(new_nr_max, BITS_PER_LONG) >
> > > +           round_up(old_nr_max, BITS_PER_LONG);
> > > +}
> > > +
> > >    static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> > >                        int map_size, int defer_size,
> > > -                    int old_map_size, int old_defer_size)
> > > +                    int old_map_size, int old_defer_size,
> > > +                    int new_nr_max)
> > >    {
> > >        struct shrinker_info *new, *old;
> > >        struct mem_cgroup_per_node *pn;
> > > @@ -240,12 +247,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> > >            if (!old)
> > >                return 0;
> > >    +        if (!need_expand(new_nr_max, old->map_nr_max))
> > > +            return 0;
> > > +
> > >            new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
> > >            if (!new)
> > >                return -ENOMEM;
> > >              new->nr_deferred = (atomic_long_t *)(new + 1);
> > >            new->map = (void *)new->nr_deferred + defer_size;
> > > +        new->map_nr_max = new_nr_max;
> > >              /* map: set all old bits, clear all new bits */
> > >            memset(new->map, (int)0xff, old_map_size);
> > > @@ -295,6 +306,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
> > >            }
> > >            info->nr_deferred = (atomic_long_t *)(info + 1);
> > >            info->map = (void *)info->nr_deferred + defer_size;
> > > +        info->map_nr_max = shrinker_nr_max;
> > >            rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> > >        }
> > >        up_write(&shrinker_rwsem);
> > > @@ -302,12 +314,6 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
> > >        return ret;
> > >    }
> > >    -static inline bool need_expand(int nr_max)
> > > -{
> > > -    return round_up(nr_max, BITS_PER_LONG) >
> > > -           round_up(shrinker_nr_max, BITS_PER_LONG);
> > > -}
> > > -
> > >    static int expand_shrinker_info(int new_id)
> > >    {
> > >        int ret = 0;
> > > @@ -316,7 +322,7 @@ static int expand_shrinker_info(int new_id)
> > >        int old_map_size, old_defer_size = 0;
> > >        struct mem_cgroup *memcg;
> > >    -    if (!need_expand(new_nr_max))
> > > +    if (!need_expand(new_nr_max, shrinker_nr_max))
> > >            goto out;
> > >          if (!root_mem_cgroup)
> > > @@ -332,7 +338,8 @@ static int expand_shrinker_info(int new_id)
> > >        memcg = mem_cgroup_iter(NULL, NULL, NULL);
> > >        do {
> > >            ret = expand_one_shrinker_info(memcg, map_size, defer_size,
> > > -                           old_map_size, old_defer_size);
> > > +                           old_map_size, old_defer_size,
> > > +                           new_nr_max);
> > >            if (ret) {
> > >                mem_cgroup_iter_break(NULL, memcg);
> > >                goto out;
> > > @@ -432,7 +439,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
> > >        for_each_node(nid) {
> > >            child_info = shrinker_info_protected(memcg, nid);
> > >            parent_info = shrinker_info_protected(parent, nid);
> > > -        for (i = 0; i < shrinker_nr_max; i++) {
> > > +        for (i = 0; i < child_info->map_nr_max; i++) {
> > >                nr = atomic_long_read(&child_info->nr_deferred[i]);
> > >                atomic_long_add(nr, &parent_info->nr_deferred[i]);
> > >            }
> > > @@ -899,7 +906,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> > >        if (unlikely(!info))
> > >            goto unlock;
> > >    -    for_each_set_bit(i, info->map, shrinker_nr_max) {
> > > +    for_each_set_bit(i, info->map, info->map_nr_max) {
> > >            struct shrink_control sc = {
> > >                .gfp_mask = gfp_mask,
> > >                .nid = nid,
>
> The patch as whole thing won't work as expected. It won't ever call shrinker with ids from [round_down(shrinker_nr_max, sizeof(unsigned long)) + 1, shrinker_nr_max - 1]
>
> Just replay the sequence we add new shrinkers:
>
> 1)We add shrinker #0:
>     shrinker_nr_max = 0;
>
>     prealloc_memcg_shrinker()
>        id = 0;
>        expand_shrinker_info(0)
>          new_nr_max = 1;
>          expand_one_shrinker_info(new_nr_max = 1)
>            new->map_nr_max = 1;
>          shrinker_nr_max = 1;
>
> 2)We add shrinker #1:
>     prealloc_memcg_shrinker()
>       id = 1;
>       expand_shrinker_info(1)
>         new_nr_max = 2;
>         need_expand(2, 1) => false => ignore expand
>         shrinker_nr_max = 2;
>
> 3)Then we call shrinker:
>    shrink_slab_memcg()
>      for_each_set_bit(i, info->map, 1/* info->map_nr_max */ ) {
>      } => ignore shrinker #1
>
> I'd fixed this patch by something like the below:
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9f895ca6216c..bb617a3871f1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -224,12 +224,6 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
>   					 lockdep_is_held(&shrinker_rwsem));
>   }
>   
> -static inline bool need_expand(int new_nr_max, int old_nr_max)
> -{
> -	return round_up(new_nr_max, BITS_PER_LONG) >
> -	       round_up(old_nr_max, BITS_PER_LONG);
> -}
> -
>   static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>   				    int map_size, int defer_size,
>   				    int old_map_size, int old_defer_size,
> @@ -247,9 +241,6 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>   		if (!old)
>   			return 0;
>   
> -		if (!need_expand(new_nr_max, old->map_nr_max))
> -			return 0;
> -

Maybe we can keep this. For example, when we failed to allocate memory 
by calling kvmalloc_node() last time, some shrinker_info may have been
expanded, and these shrinker_info do not need to be expanded again.

>   		new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
>   		if (!new)
>   			return -ENOMEM;
> @@ -317,14 +308,11 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>   static int expand_shrinker_info(int new_id)
>   {
>   	int ret = 0;
> -	int new_nr_max = new_id + 1;
> +	int new_nr_max = round_up(new_id + 1, BITS_PER_LONG);
>   	int map_size, defer_size = 0;
>   	int old_map_size, old_defer_size = 0;
>   	struct mem_cgroup *memcg;
>   
> -	if (!need_expand(new_nr_max, shrinker_nr_max))
> -		goto out;
> -
>   	if (!root_mem_cgroup)
>   		goto out;
>   
> @@ -359,9 +347,11 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
>   
>   		rcu_read_lock();
>   		info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
> -		/* Pairs with smp mb in shrink_slab() */
> -		smp_mb__before_atomic();
> -		set_bit(shrinker_id, info->map);
> +		if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
> +			/* Pairs with smp mb in shrink_slab() */
> +			smp_mb__before_atomic();
> +			set_bit(shrinker_id, info->map);
> +		}
>   		rcu_read_unlock();
>   	}
>   }
>
> (I also added a new check into set_shrinker_bit() for safety).
>
> Kirill

--
Thanks,
Qi