linux-kernel - Re: [RFC 3/3] sched/topology: Different sched groups must not have the same balance cpu

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Message-ID: <731e0515-63e8-2a58-832d-89619065a328@redhat.com>
Date:   Mon, 17 Apr 2017 12:34:05 -0300
From:   Lauro Venancio <lvenanci@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 3/3] sched/topology: Different sched groups must not have
 the same balance cpu

On 04/14/2017 01:49 PM, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 10:56:09AM -0300, Lauro Ramos Venancio wrote:
>> Currently, the group balance cpu is the groups's first CPU. But with
>> overlapping groups, two different groups can have the same first CPU.
>>
>> This patch uses the group mask to mark all the CPUs that have a
>> particular group as its main sched group. The group balance cpu is the
>> first group CPU that is also in the mask.
> Please give a NUMA configuration and CPU number where this goes wrong.
On a 4 nodes with ring topology, the groups (0-1,3 [cpu 0]),  (0-2 [cpu
1]) and (0,2-3 [cpu 3]) share the same sched_group_capacity instance
when the first groups cpu is used to select the sgc.
>
> Because only the first group of a domain matters, and with the other
> thing fixed, I'm not immediately seeing where we go wobbly.

Before patch 2, the group balance cpu was implicitly used to select the
sched_group_capacity instance. When two different groups had the same
balance cpu, they shared the same sched_group_capacity instance.

After patch 2, one different sched_group_capacity instance is assigned
to each group instance.

This patch ensures tree things:

1) different instances of the same group share the same
sched_group_capacity instance.

2) instances of different groups don't share the same
sched_group_capacity instance.

3) the group balance cpu must be one of the cpus where the group is
installed.

I am rebasing this patch on top of your patches.