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    SubjectRe: [PATCH v2 1/2] memcg: flush stats only if updated
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    Hi Michal,

    On Wed, Oct 13, 2021 at 11:01 AM Michal Koutný <mkoutny@suse.com> wrote:
    >
    > On Fri, Oct 01, 2021 at 12:00:39PM -0700, Shakeel Butt <shakeelb@google.com> wrote:
    > > In this patch we kept the stats update codepath very minimal and let the
    > > stats reader side to flush the stats only when the updates are over a
    > > specific threshold.  For now the threshold is (nr_cpus * CHARGE_BATCH).
    >
    > BTW, a noob question -- are the updates always single page sized?
    >
    > This is motivated by apples vs oranges comparison since the
    >         nr_cpus * MEMCG_CHARGE_BATCH
    > suggests what could the expected error be in pages (bytes). But it's mostly
    > wrong since: a) uncertain single-page updates, b) various counter
    > updates summed together. I wonder whether the formula can serve to
    > provide at least some (upper) estimate.
    >

    Thanks for your review. This forces me to think more on this because each
    update does not necessarily be a single page sized update e.g. adding a hugepage
    to an LRU.

    Though I think the error is time bounded by 2 seconds but in those 2 seconds
    mathematically the error can be large. What do you think of the following
    change? It will bound the error better within the 2 seconds window.



    From e87a36eedd02b0d10d8f66f83833bd6e2bae17b8 Mon Sep 17 00:00:00 2001
    From: Shakeel Butt <shakeelb@google.com>
    Date: Thu, 14 Oct 2021 08:49:06 -0700
    Subject: [PATCH] Better bounds on the stats error

    ---
    mm/memcontrol.c | 20 +++++++++++++-------
    1 file changed, 13 insertions(+), 7 deletions(-)

    diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    index 8f1d9c028897..e5d5c850a521 100644
    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -626,14 +626,20 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
    static void flush_memcg_stats_dwork(struct work_struct *w);
    static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork);
    static DEFINE_SPINLOCK(stats_flush_lock);
    -static DEFINE_PER_CPU(unsigned int, stats_updates);
    +static DEFINE_PER_CPU(int, stats_diff);
    static atomic_t stats_flush_threshold = ATOMIC_INIT(0);

    -static inline void memcg_rstat_updated(struct mem_cgroup *memcg)
    +static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
    {
    + unsigned int x;
    +
    cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
    - if (!(__this_cpu_inc_return(stats_updates) % MEMCG_CHARGE_BATCH))
    - atomic_inc(&stats_flush_threshold);
    +
    + x = abs(__this_cpu_add_return(stats_diff, val));
    + if (x > MEMCG_CHARGE_BATCH) {
    + atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold);
    + __this_cpu_write(stats_diff, 0);
    + }
    }

    static void __mem_cgroup_flush_stats(void)
    @@ -672,7 +678,7 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val)
    return;

    __this_cpu_add(memcg->vmstats_percpu->state[idx], val);
    - memcg_rstat_updated(memcg);
    + memcg_rstat_updated(memcg, val);
    }

    /* idx can be of type enum memcg_stat_item or node_stat_item. */
    @@ -705,7 +711,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
    /* Update lruvec */
    __this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);

    - memcg_rstat_updated(memcg);
    + memcg_rstat_updated(memcg, val);
    }

    /**
    @@ -807,7 +813,7 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
    return;

    __this_cpu_add(memcg->vmstats_percpu->events[idx], count);
    - memcg_rstat_updated(memcg);
    + memcg_rstat_updated(memcg, val);
    }

    static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
    --
    2.33.0.882.g93a45727a2-goog
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