linux-kernel - Re: [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology

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Date:   Thu, 13 Apr 2017 17:48:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lauro Ramos Venancio <lvenanci@...hat.com>
Cc:     linux-kernel@...r.kernel.org, lwang@...hat.com, riel@...hat.com,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC 2/3] sched/topology: fix sched groups on NUMA machines with
 mesh topology

On Thu, Apr 13, 2017 at 10:56:08AM -0300, Lauro Ramos Venancio wrote:
> Currently, on a 4 nodes NUMA machine with ring topology, two sched
> groups are generated for the last NUMA sched domain. One group has the
> CPUs from NUMA nodes 3, 0 and 1; the other group has the CPUs from nodes
> 1, 2 and 3. As CPUs from nodes 1 and 3 belongs to both groups, the
> scheduler is unable to directly move tasks between these nodes. In the
> worst scenario, when a set of tasks are bound to nodes 1 and 3, the
> performance is severely impacted because just one node is used while the
> other node remains idle.

I feel a picture would be ever so much clearer.

> This patch constructs the sched groups from each CPU perspective. So, on
> a 4 nodes machine with ring topology, while nodes 0 and 2 keep the same
> groups as before [(3, 0, 1)(1, 2, 3)], nodes 1 and 3 have new groups
> [(0, 1, 2)(2, 3, 0)]. This allows moving tasks between any node 2-hops
> apart.

So I still have no idea what specifically goes wrong and how this fixes
it. Changelog is impenetrable.

"From each CPU's persepective" doesn't really help, there already is a
for_each_cpu() in.

Also, since I'm not sure what happend to the 4 node system, I cannot
begin to imagine what would happen on the 8 node one.