Re: [PATCH v3 3/5] mm: memcg: add per-memcg zswap writeback stat

[Nhat Pham]

On Wed, Oct 18, 2023 at 4:25 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Oct 17, 2023 at 4:21 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> >
> > From: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
> >
> > Since zswap now writes back pages from memcg-specific LRUs, we now need a
> > new stat to show writebacks count for each memcg.
> >
> > Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
> > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
> > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> > ---
> >  include/linux/memcontrol.h |  2 ++
> >  mm/memcontrol.c            | 15 +++++++++++++++
> >  mm/zswap.c                 |  3 +++
> >  3 files changed, 20 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 3de10fabea0f..7868b1e00bf5 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -38,6 +38,7 @@ enum memcg_stat_item {
> >         MEMCG_KMEM,
> >         MEMCG_ZSWAP_B,
> >         MEMCG_ZSWAPPED,
> > +       MEMCG_ZSWAP_WB,
> >         MEMCG_NR_STAT,
> >  };
> >
> > @@ -1884,6 +1885,7 @@ static inline void count_objcg_event(struct obj_cgroup *objcg,
> >  bool obj_cgroup_may_zswap(struct obj_cgroup *objcg);
> >  void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size);
> >  void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size);
> > +void obj_cgroup_report_zswap_wb(struct obj_cgroup *objcg);
> >  #else
> >  static inline bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
> >  {
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 1bde67b29287..a9118871e5a6 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1505,6 +1505,7 @@ static const struct memory_stat memory_stats[] = {
> >  #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
> >         { "zswap",                      MEMCG_ZSWAP_B                   },
> >         { "zswapped",                   MEMCG_ZSWAPPED                  },
> > +       { "zswap_wb",                   MEMCG_ZSWAP_WB                  },
>
> zswap_writeback would be more consistent with file_writeback below.
>
> Taking a step back, this is not really a "state". We increment it by 1
> every time and never decrement it. Sounds awfully similar to events :)

Ah yeah, this is probably closer to zswpin/zswpout counters :)
We can probably re-use that piece of logic.

>
> You can also use count_objcg_event() directly and avoid the need for
> obj_cgroup_report_zswap_wb() below.
>
> >  #endif
> >         { "file_mapped",                NR_FILE_MAPPED                  },
> >         { "file_dirty",                 NR_FILE_DIRTY                   },
> > @@ -1541,6 +1542,7 @@ static int memcg_page_state_unit(int item)
> >         switch (item) {
> >         case MEMCG_PERCPU_B:
> >         case MEMCG_ZSWAP_B:
> > +       case MEMCG_ZSWAP_WB:
> >         case NR_SLAB_RECLAIMABLE_B:
> >         case NR_SLAB_UNRECLAIMABLE_B:
> >         case WORKINGSET_REFAULT_ANON:
> > @@ -7861,6 +7863,19 @@ void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size)
> >         rcu_read_unlock();
> >  }
> >
> > +void obj_cgroup_report_zswap_wb(struct obj_cgroup *objcg)
> > +{
> > +       struct mem_cgroup *memcg;
> > +
> > +       if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> > +               return;
> > +
> > +       rcu_read_lock();
> > +       memcg = obj_cgroup_memcg(objcg);
> > +       mod_memcg_state(memcg, MEMCG_ZSWAP_WB, 1);
> > +       rcu_read_unlock();
> > +}
> > +
> >  static u64 zswap_current_read(struct cgroup_subsys_state *css,
> >                               struct cftype *cft)
> >  {
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index d2989ad11814..15485427e3fa 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -704,6 +704,9 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
> >         }
> >         zswap_written_back_pages++;
> >
> > +       if (entry->objcg)
> > +               obj_cgroup_report_zswap_wb(entry->objcg);
> > +
> >         /*
> >          * Writeback started successfully, the page now belongs to the
> >          * swapcache. Drop the entry from zswap - unless invalidate already
> > --
> > 2.34.1