Re: [PATCH v2 1/2] memcg: flush stats only if updated

Michal Koutný
2021-10-13T18:01:30Z

On Fri, Oct 01, 2021 at 12:00:39PM -0700, Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 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.

> +static inline void memcg_rstat_updated(struct mem_cgroup *memcg)
> +{
> + cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
> + if (!(__this_cpu_inc_return(stats_updates) % MEMCG_CHARGE_BATCH))
> + atomic_inc(&stats_flush_threshold);
> +}

Neat trick! (I guess there are no benchmarks complaining about this
(yet)).

Overall,

Reviewed-by: Michal Koutný <mkoutny-IBi9RG/b67k@public.gmane.org>