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

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Message-ID: <20211013180130.GB22036@blackbody.suse.cz>
Date:   Wed, 13 Oct 2021 20:01:30 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] memcg: flush stats only if updated

On Fri, Oct 01, 2021 at 12:00:39PM -0700, Shakeel Butt <shakeelb@...gle.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.


> +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@...e.com>