linux-kernel - Re: [PATCH 1/7] mm: memcontrol: fix cpuhotplug statistics flushing

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Date:   Thu, 4 Feb 2021 14:28:52 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Roman Gushchin <guro@...com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 1/7] mm: memcontrol: fix cpuhotplug statistics flushing

On Tue 02-02-21 13:47:40, Johannes Weiner wrote:
> The memcg hotunplug callback erroneously flushes counts on the local
> CPU, not the counts of the CPU going away; those counts will be lost.
> 
> Flush the CPU that is actually going away.
> 
> Also simplify the code a bit by using mod_memcg_state() and
> count_memcg_events() instead of open-coding the upward flush - this is
> comparable to how vmstat.c handles hotunplug flushing.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Michal Hocko <mhocko@...e.com>

Shakeel has already pointed out Fixes.

> ---
>  mm/memcontrol.c | 35 +++++++++++++++++++++--------------
>  1 file changed, 21 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ed5cc78a8dbf..8120d565dd79 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2411,45 +2411,52 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
>  static int memcg_hotplug_cpu_dead(unsigned int cpu)
>  {
>  	struct memcg_stock_pcp *stock;
> -	struct mem_cgroup *memcg, *mi;
> +	struct mem_cgroup *memcg;
>  
>  	stock = &per_cpu(memcg_stock, 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++) {
>  			int nid;
> -			long x;
>  
> -			x = this_cpu_xchg(memcg->vmstats_percpu->stat[i], 0);
> -			if (x)
> -				for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
> -					atomic_long_add(x, &memcg->vmstats[i]);
> +			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;
> +				long x;
>  
>  				pn = mem_cgroup_nodeinfo(memcg, nid);
> -				x = this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
> -				if (x)
> +				lstatc = per_cpu_ptr(pn->lruvec_stat_cpu, cpu);
> +
> +				x = lstatc->count[i];
> +				lstatc->count[i] = 0;
> +
> +				if (x) {
>  					do {
>  						atomic_long_add(x, &pn->lruvec_stat[i]);
>  					} while ((pn = parent_nodeinfo(pn, nid)));
> +				}
>  			}
>  		}
>  
>  		for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
> -			long x;
> -
> -			x = this_cpu_xchg(memcg->vmstats_percpu->events[i], 0);
> -			if (x)
> -				for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
> -					atomic_long_add(x, &memcg->vmevents[i]);
> +			if (statc->events[i]) {
> +				count_memcg_events(memcg, i, statc->events[i]);
> +				statc->events[i] = 0;
> +			}
>  		}
>  	}
>  
> -- 
> 2.30.0
> 

-- 
Michal Hocko
SUSE Labs