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

[Kirill Tkhai]

On 04.02.2021 20:23, Yang Shi wrote:
> On Thu, Feb 4, 2021 at 12:42 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>>
>> 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?
> 
> What do you mean about "old places"? Where was it introduced in v5 (in
> patch #10)?

I mean existing places touched by this patch, which became to use the new helper
in this patch: free_shrinker_info(), expand_one_shrinker_info(), shrink_slab_memcg().

>>
>>> +
>>>  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()?
> 
> add_nr_deferred_memcg() sounds more self-explained to me.
> 
>>
>>>  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;
>>>
>>>
>>
>>