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

From: Peter Zijlstra <peterz@infradead.org>
To: Lauro Ramos Venancio <lvenanci@redhat.com>
Cc: linux-kernel@vger.kernel.org, lwang@redhat.com, riel@redhat.com,
	Mike Galbraith <efault@gmx.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology
Date: Fri, 14 Apr 2017 18:58:57 +0200	[thread overview]
Message-ID: <20170414165857.7n75lxk4usfsbjaq@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170414113813.vktcpsrsuu2st2fm@hirez.programming.kicks-ass.net>

On Fri, Apr 14, 2017 at 01:38:13PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 10:56:08AM -0300, Lauro Ramos Venancio wrote:
> > 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.
> 
> Ah,.. so after drawing pictures I see what went wrong; duh :-(
> 
> An equivalent patch would be (if for_each_cpu_wrap() were exposed):
> 
> @@ -521,11 +588,11 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
>  	struct cpumask *covered = sched_domains_tmpmask;
>  	struct sd_data *sdd = sd->private;
>  	struct sched_domain *sibling;
> -	int i;
> +	int i, wrap;
>  
>  	cpumask_clear(covered);
>  
> -	for_each_cpu(i, span) {
> +	for_each_cpu_wrap(i, span, cpu, wrap) {
>  		struct cpumask *sg_span;
>  
>  		if (cpumask_test_cpu(i, covered))
> 
> 
> We need to start iterating at @cpu, not start at 0 every time.
> 
> 

OK, please have a look here:

https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=sched/core