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Date:   Mon, 11 Mar 2019 13:38:25 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guroan@gmail.com>
Cc:     linux-mm@kvack.org, kernel-team@fb.com,
        linux-kernel@vger.kernel.org, Tejun Heo <tj@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Michal Hocko <mhocko@kernel.org>, Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH 5/5] mm: spill memcg percpu stats and events before
 releasing
Message-ID: <20190311173825.GE10823@cmpxchg.org>
References: <20190307230033.31975-1-guro@fb.com>
 <20190307230033.31975-6-guro@fb.com>
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On Thu, Mar 07, 2019 at 03:00:33PM -0800, Roman Gushchin wrote:
> Spill percpu stats and events data to corresponding before releasing
> percpu memory.
> 
> Although per-cpu stats are never exactly precise, dropping them on
> floor regularly may lead to an accumulation of an error. So, it's
> safer to sync them before releasing.
> 
> To minimize the number of atomic updates, let's sum all stats/events
> on all cpus locally, and then make a single update per entry.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
>  mm/memcontrol.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 18e863890392..b7eb6fac735e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4612,11 +4612,63 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>  	return 0;
>  }
>  
> +/*
> + * Spill all per-cpu stats and events into atomics.
> + * Try to minimize the number of atomic writes by gathering data from
> + * all cpus locally, and then make one atomic update.
> + * No locking is required, because no one has an access to
> + * the offlined percpu data.
> + */
> +static void mem_cgroup_spill_offlined_percpu(struct mem_cgroup *memcg)
> +{
> +	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
> +	struct lruvec_stat __percpu *lruvec_stat_cpu;
> +	struct mem_cgroup_per_node *pn;
> +	int cpu, i;
> +	long x;
> +
> +	vmstats_percpu = memcg->vmstats_percpu_offlined;
> +
> +	for (i = 0; i < MEMCG_NR_STAT; i++) {
> +		int nid;
> +
> +		x = 0;
> +		for_each_possible_cpu(cpu)
> +			x += per_cpu(vmstats_percpu->stat[i], cpu);
> +		if (x)
> +			atomic_long_add(x, &memcg->vmstats[i]);
> +
> +		if (i >= NR_VM_NODE_STAT_ITEMS)
> +			continue;
> +
> +		for_each_node(nid) {
> +			pn = mem_cgroup_nodeinfo(memcg, nid);
> +			lruvec_stat_cpu = pn->lruvec_stat_cpu_offlined;
> +
> +			x = 0;
> +			for_each_possible_cpu(cpu)
> +				x += per_cpu(lruvec_stat_cpu->count[i], cpu);
> +			if (x)
> +				atomic_long_add(x, &pn->lruvec_stat[i]);
> +		}
> +	}
> +
> +	for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
> +		x = 0;
> +		for_each_possible_cpu(cpu)
> +			x += per_cpu(vmstats_percpu->events[i], cpu);
> +		if (x)
> +			atomic_long_add(x, &memcg->vmevents[i]);
> +	}

This looks good, but couldn't this be merged with the cpu offlining?
It seems to be exactly the same code, except for the nesting of the
for_each_possible_cpu() iteration here.

This could be a function that takes a CPU argument and then iterates
the cgroups and stat items to collect and spill the counters of that
specified CPU; offlining would call it once, and this spill code here
would call it for_each_possible_cpu().

We shouldn't need the atomicity of this_cpu_xchg() during hotunplug,
the scheduler isn't even active on that CPU anymore when it's called.