linux-kernel - Re: [PATCH 04/11] mm/vmstat: Convert NUMA statistics to basic NUMA counters

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Date:   Wed, 14 Apr 2021 17:56:53 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH 04/11] mm/vmstat: Convert NUMA statistics to basic NUMA
 counters

On 4/14/21 5:18 PM, Mel Gorman wrote:
> On Wed, Apr 14, 2021 at 02:56:45PM +0200, Vlastimil Babka wrote:
>> So it seems that this intermediate assignment to zone counters (using
>> atomic_long_set() even) is unnecessary and this could mimic sum_vm_events() that
>> just does the summation on a local array?
>> 
> 
> The atomic is unnecessary for sure but using a local array is
> problematic because of your next point.

IIUC vm_events seems to do fine without a centralized array and handling CPU hot
remove at the sime time ...

>> And probably a bit more serious is that vm_events have vm_events_fold_cpu() to
>> deal with a cpu going away, but after your patch the stats counted on a cpu just
>> disapepar from the sums as it goes offline as there's no such thing for the numa
>> counters.
>> 
> 
> That is a problem I missed. Even if zonestats was preserved on
> hot-remove, fold_vm_zone_numa_events would not be reading the CPU so
> hotplug events jump all over the place.
> 
> So some periodic folding is necessary. I would still prefer not to do it
> by time but it could be done only on overflow or when a file like
> /proc/vmstat is read. I'll think about it a bit more and see what I come
> up with.

... because vm_events_fold_cpu() seems to simply move the stats from the CPU
being offlined to the current one. So the same approach should be enough for
NUMA stats?

> Thanks!
>