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

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Date:   Thu, 13 Apr 2017 11:27:49 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Lauro Ramos Venancio <lvenanci@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     lwang@...hat.com, Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Thu, 2017-04-13 at 10:56 -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.
> 

This is not your fault, but this code is really hard
to understand.

Your comments tell me what the code does, but not
really why. 

> +++ b/kernel/sched/topology.c
> @@ -477,27 +477,31 @@ enum s_alloc {
>  };
>  
>  /*
> - * Build an iteration mask that can exclude certain CPUs from the
> upwards
> - * domain traversal.
> + * An overlap sched group may not be present in all CPUs that
> compose the
> + * group. So build the mask, marking all the group CPUs where it is
> present.
>   *
>   * Asymmetric node setups can result in situations where the domain
> tree is of
>   * unequal depth, make sure to skip domains that already cover the
> entire
>   * range.
> - *
> - * In that case build_sched_domains() will have terminated the
> iteration early
> - * and our sibling sd spans will be empty. Domains should always
> include the
> - * CPU they're built on, so check that.
>   */

Why are we doing this?

Could the comment explain why things need to be
this way?


> -	for_each_cpu(i, span) {
> +	for_each_cpu(i, sg_span) {
>  		sibling = *per_cpu_ptr(sdd->sd, i);
> -		if (!cpumask_test_cpu(i,
> sched_domain_span(sibling)))
> +
> +		/*
> +		 * Asymmetric node setups: skip domains that are
> already
> +		 * done.
> +		 */
> +		if (!sibling->groups)
> +			continue;
> +

What does this mean?

I would really like it if this code was a little
better documented.  This is not something I can
put on you for most of the code, but I can at least
ask it for the code you are adding :)

The same goes for the rest of this patch.