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Date:   Wed, 29 Nov 2017 13:17:40 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Kemi Wang <kemi.wang@...el.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Johannes Weiner <hannes@...xchg.org>,
        Christopher Lameter <cl@...ux.com>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Dave <dave.hansen@...ux.intel.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Ying Huang <ying.huang@...el.com>,
        Aaron Lu <aaron.lu@...el.com>, Aubrey Li <aubrey.li@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: NUMA stats code cleanup and enhancement

On Tue 28-11-17 14:00:23, Kemi Wang wrote:
> The existed implementation of NUMA counters is per logical CPU along with
> zone->vm_numa_stat[] separated by zone, plus a global numa counter array
> vm_numa_stat[]. However, unlike the other vmstat counters, numa stats don't
> effect system's decision and are only read from /proc and /sys, it is a
> slow path operation and likely tolerate higher overhead. Additionally,
> usually nodes only have a single zone, except for node 0. And there isn't
> really any use where you need these hits counts separated by zone.
> 
> Therefore, we can migrate the implementation of numa stats from per-zone to
> per-node, and get rid of these global numa counters. It's good enough to
> keep everything in a per cpu ptr of type u64, and sum them up when need, as
> suggested by Andi Kleen. That's helpful for code cleanup and enhancement
> (e.g. save more than 130+ lines code).

I agree. Having these stats per zone is a bit of overcomplication. The
only consumer is /proc/zoneinfo and I would argue this doesn't justify
the additional complexity. Who does really need to know per zone broken
out numbers?

Anyway, I haven't checked your implementation too deeply but why don't
you simply define static percpu array for each numa node?
[...]
> +extern u64 __percpu *vm_numa_stat;
[...]
> +#ifdef CONFIG_NUMA
> +	size = sizeof(u64) * num_possible_nodes() * NR_VM_NUMA_STAT_ITEMS;
> +	align = __alignof__(u64[num_possible_nodes() * NR_VM_NUMA_STAT_ITEMS]);
> +	vm_numa_stat = (u64 __percpu *)__alloc_percpu(size, align);
> +#endif
-- 
Michal Hocko
SUSE Labs

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