linux-kernel - Re: [PATCH 1/2] mm: NUMA stats code cleanup and enhancement

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Date:   Tue, 28 Nov 2017 13:56:53 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Kemi Wang <kemi.wang@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michal Hocko <mhocko@...e.com>,
        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 Nov 2017 10:40:52 -0800 Andi Kleen <ak@...ux.intel.com> wrote:

> Vlastimil Babka <vbabka@...e.cz> writes:
> >
> > I'm worried about the "for_each_possible..." approach here and elsewhere
> > in the patch as it can be rather excessive compared to the online number
> > of cpus (we've seen BIOSes report large numbers of possible CPU's). IIRC
> 
> Even if they report a few hundred extra reading some more shared cache lines
> is very cheap. The prefetcher usually quickly figures out such a pattern
> and reads it all in parallel.
> 
> I doubt it will be noticeable, especially not in a slow path
> like reading something from proc/sys.

We say that, then a few years it comes back and bites us on our
trailing edges.

> > the general approach with vmstat is to query just online cpu's / nodes,
> > and if they go offline, transfer their accumulated stats to some other
> > "victim"?
> 
> That's very complicated, and unlikely to be worth it.

for_each_online_cpu() and a few-line hotplug handler?  I'd like to see
an implementation before deciding that it's too complex...