Re: [v5 PATCH 08/11] mm: vmscan: use per memcg nr_deferred of shrinker

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 29.01.2021 17:55, Kirill Tkhai wrote:
> On 28.01.2021 02:33, 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@gmail.com>
>> ---
>>  mm/vmscan.c | 87 ++++++++++++++++++++++++++++++++++++++++++++---------
>>  1 file changed, 73 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 20be0db291fe..e1f8960f5cf6 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -205,7 +205,8 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>  
>>  	for_each_node(nid) {
>>  		old = rcu_dereference_protected(
>> -			mem_cgroup_nodeinfo(memcg, nid)->shrinker_info, true);
>> +			mem_cgroup_nodeinfo(memcg, nid)->shrinker_info,
>> +			lockdep_is_held(&shrinker_rwsem));
> 
> Won't it better to pack this repeating pattern into helper function, e.g.:
> 
> static struct shrinker_info memcg_shrinker_info(struct mem_cgroup *memcg, int nid)
> {
> 	return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
>                                          lockdep_is_held(&shrinker_rwsem));
> }
> 
> ?
> 
> Even shrink_slab_memcg() may want to use it.

Hm, I see you already introduced a helper in [10/11], but it is used in only place.
Then, we should use it for all places (introduce the helper earlier).
 
>>  		/* Not yet online memcg */
>>  		if (!old)
>>  			return 0;
>> @@ -239,7 +240,8 @@ 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 = rcu_dereference_protected(pn->shrinker_info,
>> +						 lockdep_is_held(&shrinker_rwsem));
>>  		if (info)
>>  			kvfree(info);
>>  		rcu_assign_pointer(pn->shrinker_info, NULL);
>> @@ -360,6 +362,27 @@ 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 = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
>> +					 lockdep_is_held(&shrinker_rwsem));
>> +	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 = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
>> +					 lockdep_is_held(&shrinker_rwsem));
>> +
>> +	return atomic_long_add_return(nr, &info->nr_deferred[shrinker->id]);
>> +}
>> +
>>  static bool cgroup_reclaim(struct scan_control *sc)
>>  {
>>  	return sc->target_mem_cgroup;
>> @@ -398,6 +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;
>> @@ -409,6 +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
>> @@ -545,14 +613,10 @@ 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;
>> @@ -562,7 +626,7 @@ 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) {
>> @@ -653,14 +717,9 @@ 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;
>>
>