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SubjectRe: [PATCH 6/7] mm: memcontrol: switch to rstat
On Tue 02-02-21 13:47:45, Johannes Weiner wrote:
> Replace the memory controller's custom hierarchical stats code with
> the generic rstat infrastructure provided by the cgroup core.
>
> The current implementation does batched upward propagation from the
> write side (i.e. as stats change). The per-cpu batches introduce an
> error, which is multiplied by the number of subgroups in a tree. In
> systems with many CPUs and sizable cgroup trees, the error can be
> large enough to confuse users (e.g. 32 batch pages * 32 CPUs * 32
> subgroups results in an error of up to 128M per stat item). This can
> entirely swallow allocation bursts inside a workload that the user is
> expecting to see reflected in the statistics.
>
> In the past, we've done read-side aggregation, where a memory.stat
> read would have to walk the entire subtree and add up per-cpu
> counts. This became problematic with lazily-freed cgroups: we could
> have large subtrees where most cgroups were entirely idle. Hence the
> switch to change-driven upward propagation. Unfortunately, it needed
> to trade accuracy for speed due to the write side being so hot.
>
> Rstat combines the best of both worlds: from the write side, it
> cheaply maintains a queue of cgroups that have pending changes, so
> that the read side can do selective tree aggregation. This way the
> reported stats will always be precise and recent as can be, while the
> aggregation can skip over potentially large numbers of idle cgroups.
>
> This adds a second vmstats to struct mem_cgroup (MEMCG_NR_STAT +
> NR_VM_EVENT_ITEMS) to track pending subtree deltas during upward
> aggregation. It removes 3 words from the per-cpu data. It eliminates
> memcg_exact_page_state(), since memcg_page_state() is now exact.

The above confused me a bit. I can see the pcp data size increased by
adding _prev. The resulting memory footprint should be increased by
sizeof(long) * (MEMCG_NR_STAT + NR_VM_EVENT_ITEMS) * (CPUS + 1)
which is roughly 1kB per CPU per memcg unless I have made any
mistake. This is a quite a lot and it should be mentioned in the
changelog.

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Although the memory overhead is quite large and it scales both with
memcg count and CPUs so it can grow quite a bit I do not think this is
prohibitive. Although it would be really nice if this could be optimized
in the future.

All that being said, the code looks more manageable now.
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> include/linux/memcontrol.h | 67 ++++++-----
> mm/memcontrol.c | 224 +++++++++++++++----------------------
> 2 files changed, 133 insertions(+), 158 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 20ecdfae3289..a8c7a0ccc759 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -76,10 +76,27 @@ enum mem_cgroup_events_target {
> };
>
> struct memcg_vmstats_percpu {
> - long stat[MEMCG_NR_STAT];
> - unsigned long events[NR_VM_EVENT_ITEMS];
> - unsigned long nr_page_events;
> - unsigned long targets[MEM_CGROUP_NTARGETS];
> + /* Local (CPU and cgroup) page state & events */
> + long state[MEMCG_NR_STAT];
> + unsigned long events[NR_VM_EVENT_ITEMS];
> +
> + /* Delta calculation for lockless upward propagation */
> + long state_prev[MEMCG_NR_STAT];
> + unsigned long events_prev[NR_VM_EVENT_ITEMS];
> +
> + /* Cgroup1: threshold notifications & softlimit tree updates */
> + unsigned long nr_page_events;
> + unsigned long targets[MEM_CGROUP_NTARGETS];
> +};
> +
> +struct memcg_vmstats {
> + /* Aggregated (CPU and subtree) page state & events */
> + long state[MEMCG_NR_STAT];
> + unsigned long events[NR_VM_EVENT_ITEMS];
> +
> + /* Pending child counts during tree propagation */
> + long state_pending[MEMCG_NR_STAT];
> + unsigned long events_pending[NR_VM_EVENT_ITEMS];
> };
>
> struct mem_cgroup_reclaim_iter {
> @@ -287,8 +304,8 @@ struct mem_cgroup {
>
> MEMCG_PADDING(_pad1_);
>
> - atomic_long_t vmstats[MEMCG_NR_STAT];
> - atomic_long_t vmevents[NR_VM_EVENT_ITEMS];
> + /* memory.stat */
> + struct memcg_vmstats vmstats;
>
> /* memory.events */
> atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
> @@ -315,10 +332,6 @@ struct mem_cgroup {
> atomic_t moving_account;
> struct task_struct *move_lock_task;
>
> - /* Legacy local VM stats and events */
> - struct memcg_vmstats_percpu __percpu *vmstats_local;
> -
> - /* Subtree VM stats and events (batched updates) */
> struct memcg_vmstats_percpu __percpu *vmstats_percpu;
>
> #ifdef CONFIG_CGROUP_WRITEBACK
> @@ -942,10 +955,6 @@ static inline void mod_memcg_lruvec_state(struct lruvec *lruvec,
> local_irq_restore(flags);
> }
>
> -unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> - gfp_t gfp_mask,
> - unsigned long *total_scanned);
> -
> void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
> unsigned long count);
>
> @@ -1028,6 +1037,10 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
> void mem_cgroup_split_huge_fixup(struct page *head);
> #endif
>
> +unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> + gfp_t gfp_mask,
> + unsigned long *total_scanned);
> +
> #else /* CONFIG_MEMCG */
>
> #define MEM_CGROUP_ID_SHIFT 0
> @@ -1136,6 +1149,10 @@ static inline bool lruvec_holds_page_lru_lock(struct page *page,
> return lruvec == &pgdat->__lruvec;
> }
>
> +static inline void lruvec_memcg_debug(struct lruvec *lruvec, struct page *page)
> +{
> +}
> +
> static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
> {
> return NULL;
> @@ -1349,18 +1366,6 @@ static inline void mod_lruvec_kmem_state(void *p, enum node_stat_item idx,
> mod_node_page_state(page_pgdat(page), idx, val);
> }
>
> -static inline
> -unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> - gfp_t gfp_mask,
> - unsigned long *total_scanned)
> -{
> - return 0;
> -}
> -
> -static inline void mem_cgroup_split_huge_fixup(struct page *head)
> -{
> -}
> -
> static inline void count_memcg_events(struct mem_cgroup *memcg,
> enum vm_event_item idx,
> unsigned long count)
> @@ -1383,8 +1388,16 @@ void count_memcg_event_mm(struct mm_struct *mm, enum vm_event_item idx)
> {
> }
>
> -static inline void lruvec_memcg_debug(struct lruvec *lruvec, struct page *page)
> +static inline void mem_cgroup_split_huge_fixup(struct page *head)
> +{
> +}
> +
> +static inline
> +unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> + gfp_t gfp_mask,
> + unsigned long *total_scanned)
> {
> + return 0;
> }
> #endif /* CONFIG_MEMCG */
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2f97cb4cef6d..b205b2413186 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -757,6 +757,11 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
> return mz;
> }
>
> +static void memcg_flush_vmstats(struct mem_cgroup *memcg)
> +{
> + cgroup_rstat_flush(memcg->css.cgroup);
> +}
> +
> /**
> * __mod_memcg_state - update cgroup memory statistics
> * @memcg: the memory cgroup
> @@ -765,37 +770,17 @@ 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, threshold = MEMCG_CHARGE_BATCH;
> -
> if (mem_cgroup_disabled())
> return;
>
> - if (memcg_stat_item_in_bytes(idx))
> - threshold <<= PAGE_SHIFT;
> -
> - x = val + __this_cpu_read(memcg->vmstats_percpu->stat[idx]);
> - if (unlikely(abs(x) > threshold)) {
> - struct mem_cgroup *mi;
> -
> - /*
> - * Batch local counters to keep them in sync with
> - * the hierarchical ones.
> - */
> - __this_cpu_add(memcg->vmstats_local->stat[idx], x);
> - for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
> - atomic_long_add(x, &mi->vmstats[idx]);
> - x = 0;
> - }
> - __this_cpu_write(memcg->vmstats_percpu->stat[idx], x);
> + __this_cpu_add(memcg->vmstats_percpu->state[idx], val);
> + cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
> }
>
> -/*
> - * idx can be of type enum memcg_stat_item or node_stat_item.
> - * Keep in sync with memcg_exact_page_state().
> - */
> +/* idx can be of type enum memcg_stat_item or node_stat_item. */
> static unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
> {
> - long x = atomic_long_read(&memcg->vmstats[idx]);
> + long x = READ_ONCE(memcg->vmstats.state[idx]);
> #ifdef CONFIG_SMP
> if (x < 0)
> x = 0;
> @@ -803,17 +788,14 @@ static unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
> return x;
> }
>
> -/*
> - * idx can be of type enum memcg_stat_item or node_stat_item.
> - * Keep in sync with memcg_exact_page_state().
> - */
> +/* idx can be of type enum memcg_stat_item or node_stat_item. */
> static unsigned long memcg_page_state_local(struct mem_cgroup *memcg, int idx)
> {
> long x = 0;
> int cpu;
>
> for_each_possible_cpu(cpu)
> - x += per_cpu(memcg->vmstats_local->stat[idx], cpu);
> + x += per_cpu(memcg->vmstats_percpu->state[idx], cpu);
> #ifdef CONFIG_SMP
> if (x < 0)
> x = 0;
> @@ -936,30 +918,16 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
> void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
> unsigned long count)
> {
> - unsigned long x;
> -
> if (mem_cgroup_disabled())
> return;
>
> - x = count + __this_cpu_read(memcg->vmstats_percpu->events[idx]);
> - if (unlikely(x > MEMCG_CHARGE_BATCH)) {
> - struct mem_cgroup *mi;
> -
> - /*
> - * Batch local counters to keep them in sync with
> - * the hierarchical ones.
> - */
> - __this_cpu_add(memcg->vmstats_local->events[idx], x);
> - for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
> - atomic_long_add(x, &mi->vmevents[idx]);
> - x = 0;
> - }
> - __this_cpu_write(memcg->vmstats_percpu->events[idx], x);
> + __this_cpu_add(memcg->vmstats_percpu->events[idx], count);
> + cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
> }
>
> static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
> {
> - return atomic_long_read(&memcg->vmevents[event]);
> + return READ_ONCE(memcg->vmstats.events[event]);
> }
>
> static unsigned long memcg_events_local(struct mem_cgroup *memcg, int event)
> @@ -968,7 +936,7 @@ static unsigned long memcg_events_local(struct mem_cgroup *memcg, int event)
> int cpu;
>
> for_each_possible_cpu(cpu)
> - x += per_cpu(memcg->vmstats_local->events[event], cpu);
> + x += per_cpu(memcg->vmstats_percpu->events[event], cpu);
> return x;
> }
>
> @@ -1631,6 +1599,7 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
> *
> * Current memory state:
> */
> + memcg_flush_vmstats(memcg);
>
> for (i = 0; i < ARRAY_SIZE(memory_stats); i++) {
> u64 size;
> @@ -2450,22 +2419,11 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
> drain_stock(stock);
>
> for_each_mem_cgroup(memcg) {
> - struct memcg_vmstats_percpu *statc;
> int i;
>
> - statc = per_cpu_ptr(memcg->vmstats_percpu, cpu);
> -
> - for (i = 0; i < MEMCG_NR_STAT; i++) {
> + for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) {
> int nid;
>
> - if (statc->stat[i]) {
> - mod_memcg_state(memcg, i, statc->stat[i]);
> - statc->stat[i] = 0;
> - }
> -
> - if (i >= NR_VM_NODE_STAT_ITEMS)
> - continue;
> -
> for_each_node(nid) {
> struct batched_lruvec_stat *lstatc;
> struct mem_cgroup_per_node *pn;
> @@ -2484,13 +2442,6 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
> }
> }
> }
> -
> - for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
> - if (statc->events[i]) {
> - count_memcg_events(memcg, i, statc->events[i]);
> - statc->events[i] = 0;
> - }
> - }
> }
>
> return 0;
> @@ -3618,6 +3569,8 @@ static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
> {
> unsigned long val;
>
> + memcg_flush_vmstats(memcg);
> +
> if (mem_cgroup_is_root(memcg)) {
> val = memcg_page_state(memcg, NR_FILE_PAGES) +
> memcg_page_state(memcg, NR_ANON_MAPPED);
> @@ -3683,26 +3636,15 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
> }
> }
>
> -static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg)
> +static void memcg_flush_lruvec_page_state(struct mem_cgroup *memcg)
> {
> - unsigned long stat[MEMCG_NR_STAT] = {0};
> - struct mem_cgroup *mi;
> - int node, cpu, i;
> -
> - for_each_online_cpu(cpu)
> - for (i = 0; i < MEMCG_NR_STAT; i++)
> - stat[i] += per_cpu(memcg->vmstats_percpu->stat[i], cpu);
> -
> - for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
> - for (i = 0; i < MEMCG_NR_STAT; i++)
> - atomic_long_add(stat[i], &mi->vmstats[i]);
> + int node;
>
> for_each_node(node) {
> struct mem_cgroup_per_node *pn = memcg->nodeinfo[node];
> + unsigned long stat[NR_VM_NODE_STAT_ITEMS] = {0, };
> struct mem_cgroup_per_node *pi;
> -
> - for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> - stat[i] = 0;
> + int cpu, i;
>
> for_each_online_cpu(cpu)
> for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> @@ -3715,25 +3657,6 @@ static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg)
> }
> }
>
> -static void memcg_flush_percpu_vmevents(struct mem_cgroup *memcg)
> -{
> - unsigned long events[NR_VM_EVENT_ITEMS];
> - struct mem_cgroup *mi;
> - int cpu, i;
> -
> - for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
> - events[i] = 0;
> -
> - for_each_online_cpu(cpu)
> - for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
> - events[i] += per_cpu(memcg->vmstats_percpu->events[i],
> - cpu);
> -
> - for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
> - for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
> - atomic_long_add(events[i], &mi->vmevents[i]);
> -}
> -
> #ifdef CONFIG_MEMCG_KMEM
> static int memcg_online_kmem(struct mem_cgroup *memcg)
> {
> @@ -4050,6 +3973,8 @@ static int memcg_numa_stat_show(struct seq_file *m, void *v)
> int nid;
> struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
>
> + memcg_flush_vmstats(memcg);
> +
> for (stat = stats; stat < stats + ARRAY_SIZE(stats); stat++) {
> seq_printf(m, "%s=%lu", stat->name,
> mem_cgroup_nr_lru_pages(memcg, stat->lru_mask,
> @@ -4120,6 +4045,8 @@ static int memcg_stat_show(struct seq_file *m, void *v)
>
> BUILD_BUG_ON(ARRAY_SIZE(memcg1_stat_names) != ARRAY_SIZE(memcg1_stats));
>
> + memcg_flush_vmstats(memcg);
> +
> for (i = 0; i < ARRAY_SIZE(memcg1_stats); i++) {
> unsigned long nr;
>
> @@ -4596,22 +4523,6 @@ struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
> return &memcg->cgwb_domain;
> }
>
> -/*
> - * idx can be of type enum memcg_stat_item or node_stat_item.
> - * Keep in sync with memcg_exact_page().
> - */
> -static unsigned long memcg_exact_page_state(struct mem_cgroup *memcg, int idx)
> -{
> - long x = atomic_long_read(&memcg->vmstats[idx]);
> - int cpu;
> -
> - for_each_online_cpu(cpu)
> - x += per_cpu_ptr(memcg->vmstats_percpu, cpu)->stat[idx];
> - if (x < 0)
> - x = 0;
> - return x;
> -}
> -
> /**
> * mem_cgroup_wb_stats - retrieve writeback related stats from its memcg
> * @wb: bdi_writeback in question
> @@ -4637,13 +4548,14 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
> struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
> struct mem_cgroup *parent;
>
> - *pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
> + memcg_flush_vmstats(memcg);
>
> - *pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
> - *pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
> - memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
> - *pheadroom = PAGE_COUNTER_MAX;
> + *pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
> + *pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
> + *pfilepages = memcg_page_state(memcg, NR_INACTIVE_FILE) +
> + memcg_page_state(memcg, NR_ACTIVE_FILE);
>
> + *pheadroom = PAGE_COUNTER_MAX;
> while ((parent = parent_mem_cgroup(memcg))) {
> unsigned long ceiling = min(READ_ONCE(memcg->memory.max),
> READ_ONCE(memcg->memory.high));
> @@ -5275,7 +5187,6 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
> for_each_node(node)
> free_mem_cgroup_per_node_info(memcg, node);
> free_percpu(memcg->vmstats_percpu);
> - free_percpu(memcg->vmstats_local);
> kfree(memcg);
> }
>
> @@ -5283,11 +5194,10 @@ static void mem_cgroup_free(struct mem_cgroup *memcg)
> {
> memcg_wb_domain_exit(memcg);
> /*
> - * Flush percpu vmstats and vmevents to guarantee the value correctness
> - * on parent's and all ancestor levels.
> + * Flush percpu lruvec stats to guarantee the value
> + * correctness on parent's and all ancestor levels.
> */
> - memcg_flush_percpu_vmstats(memcg);
> - memcg_flush_percpu_vmevents(memcg);
> + memcg_flush_lruvec_page_state(memcg);
> __mem_cgroup_free(memcg);
> }
>
> @@ -5314,11 +5224,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
> goto fail;
> }
>
> - memcg->vmstats_local = alloc_percpu_gfp(struct memcg_vmstats_percpu,
> - GFP_KERNEL_ACCOUNT);
> - if (!memcg->vmstats_local)
> - goto fail;
> -
> memcg->vmstats_percpu = alloc_percpu_gfp(struct memcg_vmstats_percpu,
> GFP_KERNEL_ACCOUNT);
> if (!memcg->vmstats_percpu)
> @@ -5518,6 +5423,62 @@ static void mem_cgroup_css_reset(struct cgroup_subsys_state *css)
> memcg_wb_domain_size_changed(memcg);
> }
>
> +static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
> +{
> + struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> + struct mem_cgroup *parent = parent_mem_cgroup(memcg);
> + struct memcg_vmstats_percpu *statc;
> + long delta, v;
> + int i;
> +
> + statc = per_cpu_ptr(memcg->vmstats_percpu, cpu);
> +
> + for (i = 0; i < MEMCG_NR_STAT; i++) {
> + /*
> + * Collect the aggregated propagation counts of groups
> + * below us. We're in a per-cpu loop here and this is
> + * a global counter, so the first cycle will get them.
> + */
> + delta = memcg->vmstats.state_pending[i];
> + if (delta)
> + memcg->vmstats.state_pending[i] = 0;
> +
> + /* Add CPU changes on this level since the last flush */
> + v = READ_ONCE(statc->state[i]);
> + if (v != statc->state_prev[i]) {
> + delta += v - statc->state_prev[i];
> + statc->state_prev[i] = v;
> + }
> +
> + if (!delta)
> + continue;
> +
> + /* Aggregate counts on this level and propagate upwards */
> + memcg->vmstats.state[i] += delta;
> + if (parent)
> + parent->vmstats.state_pending[i] += delta;
> + }
> +
> + for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
> + delta = memcg->vmstats.events_pending[i];
> + if (delta)
> + memcg->vmstats.events_pending[i] = 0;
> +
> + v = READ_ONCE(statc->events[i]);
> + if (v != statc->events_prev[i]) {
> + delta += v - statc->events_prev[i];
> + statc->events_prev[i] = v;
> + }
> +
> + if (!delta)
> + continue;
> +
> + memcg->vmstats.events[i] += delta;
> + if (parent)
> + parent->vmstats.events_pending[i] += delta;
> + }
> +}
> +
> #ifdef CONFIG_MMU
> /* Handlers for move charge at task migration. */
> static int mem_cgroup_do_precharge(unsigned long count)
> @@ -6571,6 +6532,7 @@ struct cgroup_subsys memory_cgrp_subsys = {
> .css_released = mem_cgroup_css_released,
> .css_free = mem_cgroup_css_free,
> .css_reset = mem_cgroup_css_reset,
> + .css_rstat_flush = mem_cgroup_css_rstat_flush,
> .can_attach = mem_cgroup_can_attach,
> .cancel_attach = mem_cgroup_cancel_attach,
> .post_attach = mem_cgroup_move_task,
> --
> 2.30.0

--
Michal Hocko
SUSE Labs

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