Re: [v6 PATCH 08/11] mm: vmscan: use per memcg nr_deferred of shrinker

[Kirill Tkhai]

On 03.02.2021 20:20, Yang Shi wrote:
> Use per memcg's nr_deferred for memcg aware shrinkers.  The shrinker's nr_deferred
> will be used in the following cases:
>     1. Non memcg aware shrinkers
>     2. !CONFIG_MEMCG
>     3. memcg is disabled by boot parameter
> 
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
>  mm/vmscan.c | 94 +++++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 77 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d9126f12890f..545422d2aeec 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -190,6 +190,13 @@ static int shrinker_nr_max;
>  #define NR_MAX_TO_SHR_MAP_SIZE(nr_max) \
>  	(DIV_ROUND_UP(nr_max, BITS_PER_LONG) * sizeof(unsigned long))
>  
> +static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
> +						     int nid)
> +{
> +	return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
> +					 lockdep_is_held(&shrinker_rwsem));
> +}

Thanks for the helper. Why not to introduce and become to use it in old places
in a separate patch?

> +
>  static void free_shrinker_info_rcu(struct rcu_head *head)
>  {
>  	kvfree(container_of(head, struct shrinker_info, rcu));
> @@ -204,8 +211,7 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>  	int size = m_size + d_size;
>  
>  	for_each_node(nid) {
> -		old = rcu_dereference_protected(
> -			mem_cgroup_nodeinfo(memcg, nid)->shrinker_info, true);
> +		old = shrinker_info_protected(memcg, nid);
>  		/* Not yet online memcg */
>  		if (!old)
>  			return 0;
> @@ -239,7 +245,7 @@ void free_shrinker_info(struct mem_cgroup *memcg)
>  
>  	for_each_node(nid) {
>  		pn = mem_cgroup_nodeinfo(memcg, nid);
> -		info = rcu_dereference_protected(pn->shrinker_info, true);
> +		info = shrinker_info_protected(memcg, nid);
>  		kvfree(info);
>  		rcu_assign_pointer(pn->shrinker_info, NULL);
>  	}
> @@ -358,6 +364,25 @@ static void unregister_memcg_shrinker(struct shrinker *shrinker)
>  	up_write(&shrinker_rwsem);
>  }
>  
> +
> +static long count_nr_deferred_memcg(int nid, struct shrinker *shrinker,
> +				    struct mem_cgroup *memcg)
> +{
> +	struct shrinker_info *info;
> +
> +	info = shrinker_info_protected(memcg, nid);
> +	return atomic_long_xchg(&info->nr_deferred[shrinker->id], 0);
> +}
> +
> +static long set_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
> +				  struct mem_cgroup *memcg)
> +{
> +	struct shrinker_info *info;
> +
> +	info = shrinker_info_protected(memcg, nid);
> +	return atomic_long_add_return(nr, &info->nr_deferred[shrinker->id]);
> +}

Names confuse me a little bit. What about xchg_nr_deferred_memcg() and add_nr_deferred_memcg()?

>  static bool cgroup_reclaim(struct scan_control *sc)
>  {
>  	return sc->target_mem_cgroup;
> @@ -396,6 +421,18 @@ static void unregister_memcg_shrinker(struct shrinker *shrinker)
>  {
>  }
>  
> +static long count_nr_deferred_memcg(int nid, struct shrinker *shrinker,
> +				    struct mem_cgroup *memcg)
> +{
> +	return 0;
> +}
> +
> +static long set_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
> +				  struct mem_cgroup *memcg)
> +{
> +	return 0;
> +}
> +
>  static bool cgroup_reclaim(struct scan_control *sc)
>  {
>  	return false;
> @@ -407,6 +444,39 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  }
>  #endif
>  
> +static long count_nr_deferred(struct shrinker *shrinker,
> +			      struct shrink_control *sc)
> +{
> +	int nid = sc->nid;
> +
> +	if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> +		nid = 0;
> +
> +	if (sc->memcg &&
> +	    (shrinker->flags & SHRINKER_MEMCG_AWARE))
> +		return count_nr_deferred_memcg(nid, shrinker,
> +					       sc->memcg);
> +
> +	return atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
> +}
> +
> +
> +static long set_nr_deferred(long nr, struct shrinker *shrinker,
> +			    struct shrink_control *sc)
> +{
> +	int nid = sc->nid;
> +
> +	if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> +		nid = 0;
> +
> +	if (sc->memcg &&
> +	    (shrinker->flags & SHRINKER_MEMCG_AWARE))
> +		return set_nr_deferred_memcg(nr, nid, shrinker,
> +					     sc->memcg);
> +
> +	return atomic_long_add_return(nr, &shrinker->nr_deferred[nid]);
> +}
> +
>  /*
>   * This misses isolated pages which are not accounted for to save counters.
>   * As the data only determines if reclaim or compaction continues, it is
> @@ -539,14 +609,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	long freeable;
>  	long nr;
>  	long new_nr;
> -	int nid = shrinkctl->nid;
>  	long batch_size = shrinker->batch ? shrinker->batch
>  					  : SHRINK_BATCH;
>  	long scanned = 0, next_deferred;
>  
> -	if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> -		nid = 0;
> -
>  	freeable = shrinker->count_objects(shrinker, shrinkctl);
>  	if (freeable == 0 || freeable == SHRINK_EMPTY)
>  		return freeable;
> @@ -556,7 +622,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	 * and zero it so that other concurrent shrinker invocations
>  	 * don't also do this scanning work.
>  	 */
> -	nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
> +	nr = count_nr_deferred(shrinker, shrinkctl);
>  
>  	total_scan = nr;
>  	if (shrinker->seeks) {
> @@ -647,14 +713,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  		next_deferred = 0;
>  	/*
>  	 * move the unused scan count back into the shrinker in a
> -	 * manner that handles concurrent updates. If we exhausted the
> -	 * scan, there is no need to do an update.
> +	 * manner that handles concurrent updates.
>  	 */
> -	if (next_deferred > 0)
> -		new_nr = atomic_long_add_return(next_deferred,
> -						&shrinker->nr_deferred[nid]);
> -	else
> -		new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
> +	new_nr = set_nr_deferred(next_deferred, shrinker, shrinkctl);
>  
>  	trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan);
>  	return freed;
> @@ -674,8 +735,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>  	if (!down_read_trylock(&shrinker_rwsem))
>  		return 0;
>  
> -	info = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
> -					 true);
> +	info = shrinker_info_protected(memcg, nid);
>  	if (unlikely(!info))
>  		goto unlock;
>  
>