linux-kernel - Re: [PATCH 4/4] sched/topology: the group balance cpu must be a cpu where the group is installed

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Date:   Tue, 25 Apr 2017 17:52:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lauro Venancio <lvenanci@...hat.com>
Cc:     lwang@...hat.com, riel@...hat.com, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] sched/topology: the group balance cpu must be a cpu
 where the group is installed

On Tue, Apr 25, 2017 at 05:39:37PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 25, 2017 at 05:27:03PM +0200, Peter Zijlstra wrote:

> > Ah! the asymmetric setup, where @sibling is entirely uninitialized for
> > the top domain.

Like so then...

--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -509,6 +509,11 @@ static void build_group_mask(struct sche
 	for_each_cpu(i, sg_span) {
 		sibling = *per_cpu_ptr(sdd->sd, i);
 
+		/*
+		 * Can happen in the asymmetric case, where these siblings are
+		 * unused. The mask will not be empty because those CPUs that
+		 * do have the top domain _should_ span the domain.
+		 */
 		if (!sibling->child)
 			continue;
 
@@ -518,6 +523,9 @@ static void build_group_mask(struct sche
 
 		cpumask_set_cpu(i, sched_group_mask(sg));
 	}
+
+	/* We must not have empty masks here */
+	WARN_ON_ONCE(cpumask_empty(sched_group_mask(sg)));
 }
 
 /*