Re: [PATCH v6 02/19] mm: memcg: prepare for byte-sized vmstat items

[Roman Gushchin <guro@fb.com>]

On Tue, Jun 16, 2020 at 07:57:54PM -0700, Shakeel Butt wrote:
> On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@fb.com> wrote:
> >
> > To implement per-object slab memory accounting, we need to
> > convert slab vmstat counters to bytes. Actually, out of
> > 4 levels of counters: global, per-node, per-memcg and per-lruvec
> > only two last levels will require byte-sized counters.
> > It's because global and per-node counters will be counting the
> > number of slab pages, and per-memcg and per-lruvec will be
> > counting the amount of memory taken by charged slab objects.
> >
> > Converting all vmstat counters to bytes or even all slab
> > counters to bytes would introduce an additional overhead.
> > So instead let's store global and per-node counters
> > in pages, and memcg and lruvec counters in bytes.
> >
> > To make the API clean all access helpers (both on the read
> > and write sides) are dealing with bytes.
> >
> 
> The "dealing with bytes" is only for slab stats or all vmstat stats?

Only slab stats as now.
I've sent a percpu memory accounting patchset separately, which
will add another byte-sized counter.

> 
> > To avoid back-and-forth conversions a new flavor of read-side
> > helpers is introduced, which always returns values in pages:
> > node_page_state_pages() and global_node_page_state_pages().
> >
> > Actually new helpers are just reading raw values. Old helpers are
> > simple wrappers, which will complain on an attempt to read
> > byte value, because at the moment no one actually needs bytes.
> >
> > Thanks to Johannes Weiner for the idea of having the byte-sized API
> > on top of the page-sized internal storage.
> >
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> > ---
> >  drivers/base/node.c    |  2 +-
> >  include/linux/mmzone.h | 10 ++++++++++
> >  include/linux/vmstat.h | 14 +++++++++++++-
> >  mm/memcontrol.c        | 14 ++++++++++----
> >  mm/vmstat.c            | 30 ++++++++++++++++++++++++++----
> >  5 files changed, 60 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/base/node.c b/drivers/base/node.c
> > index 5b02f69769e8..e21e31359297 100644
> > --- a/drivers/base/node.c
> > +++ b/drivers/base/node.c
> > @@ -513,7 +513,7 @@ static ssize_t node_read_vmstat(struct device *dev,
> >
> >         for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> >                 n += sprintf(buf+n, "%s %lu\n", node_stat_name(i),
> > -                            node_page_state(pgdat, i));
> > +                            node_page_state_pages(pgdat, i));
> >
> >         return n;
> >  }
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index c4c37fd12104..fa8eb49d9898 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -206,6 +206,16 @@ enum node_stat_item {
> >         NR_VM_NODE_STAT_ITEMS
> >  };
> >
> > +/*
> > + * Returns true if the value is measured in bytes (most vmstat values are
> > + * measured in pages). This defines the API part, the internal representation
> > + * might be different.
> > + */
> > +static __always_inline bool vmstat_item_in_bytes(enum node_stat_item item)
> > +{
> > +       return false;
> > +}
> > +
> >  /*
> >   * We do arithmetic on the LRU lists in various places in the code,
> >   * so it is important to keep the active lists LRU_ACTIVE higher in
> > diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> > index aa961088c551..91220ace31da 100644
> > --- a/include/linux/vmstat.h
> > +++ b/include/linux/vmstat.h
> > @@ -8,6 +8,7 @@
> >  #include <linux/vm_event_item.h>
> >  #include <linux/atomic.h>
> >  #include <linux/static_key.h>
> > +#include <linux/mmdebug.h>
> >
> >  extern int sysctl_stat_interval;
> >
> > @@ -192,7 +193,8 @@ static inline unsigned long global_zone_page_state(enum zone_stat_item item)
> >         return x;
> >  }
> >
> > -static inline unsigned long global_node_page_state(enum node_stat_item item)
> > +static inline
> > +unsigned long global_node_page_state_pages(enum node_stat_item item)
> >  {
> >         long x = atomic_long_read(&vm_node_stat[item]);
> >  #ifdef CONFIG_SMP
> > @@ -202,6 +204,13 @@ static inline unsigned long global_node_page_state(enum node_stat_item item)
> >         return x;
> >  }
> >
> > +static inline unsigned long global_node_page_state(enum node_stat_item item)
> > +{
> > +       VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
> > +
> > +       return global_node_page_state_pages(item);
> > +}
> > +
> >  static inline unsigned long zone_page_state(struct zone *zone,
> >                                         enum zone_stat_item item)
> >  {
> > @@ -242,9 +251,12 @@ extern unsigned long sum_zone_node_page_state(int node,
> >  extern unsigned long sum_zone_numa_state(int node, enum numa_stat_item item);
> >  extern unsigned long node_page_state(struct pglist_data *pgdat,
> >                                                 enum node_stat_item item);
> > +extern unsigned long node_page_state_pages(struct pglist_data *pgdat,
> > +                                          enum node_stat_item item);
> >  #else
> >  #define sum_zone_node_page_state(node, item) global_zone_page_state(item)
> >  #define node_page_state(node, item) global_node_page_state(item)
> > +#define node_page_state_pages(node, item) global_node_page_state_pages(item)
> >  #endif /* CONFIG_NUMA */
> >
> >  #ifdef CONFIG_SMP
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index e8a91e98556b..07d02e61a73e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -681,13 +681,16 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
> >   */
> >  void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val)
> >  {
> > -       long x;
> > +       long x, threshold = MEMCG_CHARGE_BATCH;
> >
> >         if (mem_cgroup_disabled())
> >                 return;
> >
> > +       if (vmstat_item_in_bytes(idx))
> > +               threshold <<= PAGE_SHIFT;
> > +
> 
> From the above am I understanding correctly that even after moving to
> byte-level accounting, we can still see stats with potential error
> limited by (BATCH-1)*PAGE_SIZE*nr_cpus?
> 
> >         x = val + __this_cpu_read(memcg->vmstats_percpu->stat[idx]);
> > -       if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
> > +       if (unlikely(abs(x) > threshold)) {
> >                 struct mem_cgroup *mi;
> >
> >                 /*
> > @@ -718,7 +721,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  {
> >         struct mem_cgroup_per_node *pn;
> >         struct mem_cgroup *memcg;
> > -       long x;
> > +       long x, threshold = MEMCG_CHARGE_BATCH;
> >
> >         pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> >         memcg = pn->memcg;
> > @@ -729,8 +732,11 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >         /* Update lruvec */
> >         __this_cpu_add(pn->lruvec_stat_local->count[idx], val);
> >
> > +       if (vmstat_item_in_bytes(idx))
> > +               threshold <<= PAGE_SHIFT;
> > +
> >         x = val + __this_cpu_read(pn->lruvec_stat_cpu->count[idx]);
> > -       if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
> > +       if (unlikely(abs(x) > threshold)) {
> >                 pg_data_t *pgdat = lruvec_pgdat(lruvec);
> >                 struct mem_cgroup_per_node *pi;
> >
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 80c9b6221535..f1c321e1d6d3 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -341,6 +341,11 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
> >         long x;
> >         long t;
> >
> > +       if (vmstat_item_in_bytes(item)) {
> > +               VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1));
> > +               delta >>= PAGE_SHIFT;
> > +       }
> > +
> >         x = delta + __this_cpu_read(*p);
> >
> >         t = __this_cpu_read(pcp->stat_threshold);
> > @@ -398,6 +403,8 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
> >         s8 __percpu *p = pcp->vm_node_stat_diff + item;
> >         s8 v, t;
> >
> > +       VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
> > +
> >         v = __this_cpu_inc_return(*p);
> >         t = __this_cpu_read(pcp->stat_threshold);
> >         if (unlikely(v > t)) {
> > @@ -442,6 +449,8 @@ void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
> >         s8 __percpu *p = pcp->vm_node_stat_diff + item;
> >         s8 v, t;
> >
> > +       VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
> > +
> >         v = __this_cpu_dec_return(*p);
> >         t = __this_cpu_read(pcp->stat_threshold);
> >         if (unlikely(v < - t)) {
> > @@ -541,6 +550,11 @@ static inline void mod_node_state(struct pglist_data *pgdat,
> >         s8 __percpu *p = pcp->vm_node_stat_diff + item;
> >         long o, n, t, z;
> >
> > +       if (vmstat_item_in_bytes(item)) {
> > +               VM_WARN_ON_ONCE(delta & (PAGE_SIZE - 1));
> > +               delta >>= PAGE_SHIFT;
> > +       }
> > +
> >         do {
> >                 z = 0;  /* overflow to node counters */
> >
> > @@ -989,8 +1003,8 @@ unsigned long sum_zone_numa_state(int node,
> >  /*
> >   * Determine the per node value of a stat item.
> >   */
> > -unsigned long node_page_state(struct pglist_data *pgdat,
> > -                               enum node_stat_item item)
> > +unsigned long node_page_state_pages(struct pglist_data *pgdat,
> > +                                   enum node_stat_item item)
> >  {
> >         long x = atomic_long_read(&pgdat->vm_stat[item]);
> >  #ifdef CONFIG_SMP
> > @@ -999,6 +1013,14 @@ unsigned long node_page_state(struct pglist_data *pgdat,
> >  #endif
> >         return x;
> >  }
> > +
> > +unsigned long node_page_state(struct pglist_data *pgdat,
> > +                             enum node_stat_item item)
> > +{
> > +       VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
> > +
> > +       return node_page_state_pages(pgdat, item);
> > +}
> 
> So, for non-slab, node_page_state and node_page_state_pages will be
> the same but different for slab vmstats. However we should not be
> calling node_page_state with slab vmstats because we don't need it,
> right?

Right.