linux-kernel - Re: [PATCH 0/3] Recalculate per-cpu page allocator batch and high limits after deferred meminit

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Date:   Fri, 18 Oct 2019 15:48:24 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Recalculate per-cpu page allocator batch and high
 limits after deferred meminit

On Fri, 18 Oct, at 01:54:49PM, Mel Gorman wrote:
> On Fri, Oct 18, 2019 at 12:58:49PM +0100, Matt Fleming wrote:
> > On Fri, 18 Oct, at 11:56:03AM, Mel Gorman wrote:
> > > A private report stated that system CPU usage was excessive on an AMD
> > > EPYC 2 machine while building kernels with much longer build times than
> > > expected. The issue is partially explained by high zone lock contention
> > > due to the per-cpu page allocator batch and high limits being calculated
> > > incorrectly. This series addresses a large chunk of the problem. Patch 1
> > > is mostly cosmetic but prepares for patch 2 which is the real fix. Patch
> > > 3 is definiely cosmetic but was noticed while implementing the fix. Proper
> > > details are in the changelog for patch 2.
> > > 
> > >  include/linux/mm.h |  3 ---
> > >  mm/internal.h      |  3 +++
> > >  mm/page_alloc.c    | 33 ++++++++++++++++++++-------------
> > >  3 files changed, 23 insertions(+), 16 deletions(-)
> > 
> > Just to confirm, these patches don't fix the issue we're seeing on the
> > EPYC 2 machines, but they do return the batch sizes to sensible values.
> 
> To be clear, does the patch a) fix *some* of the issue and there is
> something else also going on that needs to be chased down or b) has no
> impact on build time or system CPU usage on your machine?

Sorry, I realise my email was pretty unclear.

These patches *do* fix some of the issue because I no longer see as
much contention on the zone locks with the patches applied.