Re: [v2 PATCH 6/9] mm: vmscan: use per memcg nr_deferred of shrinker

[Yang Shi]

On Mon, Dec 14, 2020 at 6:46 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>
> On Mon, Dec 14, 2020 at 02:37:19PM -0800, 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>
>
> Lots of lines way over 80 columns.

I thought that has been lifted to 100 columns.

>
> > ---
> >  mm/vmscan.c | 94 ++++++++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 83 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index bf34167dd67e..bce8cf44eca2 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -203,6 +203,12 @@ DECLARE_RWSEM(shrinker_rwsem);
> >  static DEFINE_IDR(shrinker_idr);
> >  static int shrinker_nr_max;
> >
> > +static inline bool is_deferred_memcg_aware(struct shrinker *shrinker)
> > +{
> > +     return (shrinker->flags & SHRINKER_MEMCG_AWARE) &&
> > +             !mem_cgroup_disabled();
> > +}
>
> Why do we care if mem_cgroup_disabled() is disabled here? The return
> of this function is then && sc->memcg, so if memcgs are disabled,
> sc->memcg will never be set and this mem_cgroup_disabled() check is
> completely redundant, right?

Yes, correct. I missed this point.

>
> > +
> >  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >  {
> >       int id, ret = -ENOMEM;
> > @@ -271,7 +277,58 @@ static bool writeback_throttling_sane(struct scan_control *sc)
> >  #endif
> >       return false;
> >  }
> > +
> > +static inline long count_nr_deferred(struct shrinker *shrinker,
> > +                                  struct shrink_control *sc)
> > +{
> > +     bool per_memcg_deferred = is_deferred_memcg_aware(shrinker) && sc->memcg;
> > +     struct memcg_shrinker_deferred *deferred;
> > +     struct mem_cgroup *memcg = sc->memcg;
> > +     int nid = sc->nid;
> > +     int id = shrinker->id;
> > +     long nr;
> > +
> > +     if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> > +             nid = 0;
> > +
> > +     if (per_memcg_deferred) {
> > +             deferred = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_deferred,
> > +                                                  true);
> > +             nr = atomic_long_xchg(&deferred->nr_deferred[id], 0);
> > +     } else
> > +             nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
> > +
> > +     return nr;
> > +}
> > +
> > +static inline long set_nr_deferred(long nr, struct shrinker *shrinker,
> > +                                struct shrink_control *sc)
> > +{
> > +     bool per_memcg_deferred = is_deferred_memcg_aware(shrinker) && sc->memcg;
> > +     struct memcg_shrinker_deferred *deferred;
> > +     struct mem_cgroup *memcg = sc->memcg;
> > +     int nid = sc->nid;
> > +     int id = shrinker->id;
>
> Oh, that's a nasty trap. Nobody knows if you mean "id" or "nid" in
> any of the code and a single letter type results in a bug.

Sure, will come up with more descriptive names. Maybe "nid" and "shrinker_id"?

>
> > +     long new_nr;
> > +
> > +     if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> > +             nid = 0;
> > +
> > +     if (per_memcg_deferred) {
> > +             deferred = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_deferred,
> > +                                                  true);
> > +             new_nr = atomic_long_add_return(nr, &deferred->nr_deferred[id]);
> > +     } else
> > +             new_nr = atomic_long_add_return(nr, &shrinker->nr_deferred[nid]);
> > +
> > +     return new_nr;
> > +}
> >  #else
> > +static inline bool is_deferred_memcg_aware(struct shrinker *shrinker)
> > +{
> > +     return false;
> > +}
> > +
> >  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >  {
> >       return 0;
> > @@ -290,6 +347,29 @@ static bool writeback_throttling_sane(struct scan_control *sc)
> >  {
> >       return true;
> >  }
> > +
> > +static inline long count_nr_deferred(struct shrinker *shrinker,
> > +                                  struct shrink_control *sc)
> > +{
> > +     int nid = sc->nid;
> > +
> > +     if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> > +             nid = 0;
> > +
> > +     return atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
> > +}
> > +
> > +static inline 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;
> > +
> > +     return atomic_long_add_return(nr,
> > +                                   &shrinker->nr_deferred[nid]);
> > +}
> >  #endif
>
> This is pretty ... verbose. It doesn't need to be this complex at
> all, and you shouldn't be duplicating code in multiple places. THere
> is also no need for any of these to be "inline" functions. The
> compiler will do that for static functions automatically if it makes
> sense.
>
> Ok, so you only do the memcg nr_deferred thing if NUMA_AWARE &&
> sc->memcg is true. so....
>
> static long shrink_slab_set_nr_deferred_memcg(...)
> {
>         int nid = sc->nid;
>
>         deferred = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_deferred,
>                                              true);
>         return atomic_long_add_return(nr, &deferred->nr_deferred[id]);
> }
>
> static long shrink_slab_set_nr_deferred(...)
> {
>         int nid = sc->nid;
>
>         if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
>                 nid = 0;
>         else if (sc->memcg)
>                 return shrink_slab_set_nr_deferred_memcg(...., nid);
>
>         return atomic_long_add_return(nr, &shrinker->nr_deferred[nid]);
> }
>
> And now there's no duplicated code.

Thanks for the suggestion. Will incorporate in v3.

>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx