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

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Date:   Thu, 15 Apr 2021 11:06:50 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
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 Wed, Apr 14, 2021 at 05:56:53PM +0200, Vlastimil Babka wrote:
> 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 ...
> 

The vm_events are more global in nature. They are not reported
to userspace on a per-zone (/proc/zoneinfo) basis or per-node
(/sys/devices/system/node/node*/numastat) basis so they are not equivalent.

> >> 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?
> 

Yes, or at least very similar.

-- 
Mel Gorman
SUSE Labs