linux-kernel - [RFC PATCH 0/3] Revisit NUMA imbalance tolerance and fork balancing

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Date:   Tue, 17 Nov 2020 13:42:19 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: [RFC PATCH 0/3] Revisit NUMA imbalance tolerance and fork balancing

When NUMA and CPU balancing were reconciled, there was an attempt to allow
a degree of imbalance but it caused more problems than it solved. Instead,
imbalance was only allowed with an almost idle NUMA domain. A lot of the
problems have since been addressed so it's time for a revisit. There is
also an issue with how fork is balanced across threads. It's mentioned
in this context as patch 2 and 3 should share similar behaviour in terms
of a nodes utilisation.

Patch 1 is just a cosmetic rename

Patch 2 allows a "floating" imbalance to exist so communicating tasks can
	remain on the same domain until utilisation is higher. It aims
	to balance compute availability with memory bandwidth.

Patch 3 is the interesting one. Currently fork can allow a NUMA node
	to be completely utilised as long as there are idle CPUs until
	the load balancer gets involved. This caused serious problems
	with a real workload that unfortunately I cannot share many
	details about but there is a proxy reproducer.

 kernel/sched/fair.c | 41 ++++++++++++++++++++++++-----------------
 1 file changed, 24 insertions(+), 17 deletions(-)