linux-kernel - [PATCH v5 16/19] arch_topology: Limit span of cpu_clustergroup

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Message-Id: <20220627165047.336669-17-sudeep.holla@arm.com>
Date:   Mon, 27 Jun 2022 17:50:44 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     linux-kernel@...r.kernel.org
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Atish Patra <atishp@...shpatra.org>,
        Atish Patra <atishp@...osinc.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Qing Wang <wangqing@...o.com>,
        Rob Herring <robh+dt@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Pierre Gondois <pierre.gondois@....com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org,
        Darren Hart <darren@...amperecomputing.com>
Subject: [PATCH v5 16/19] arch_topology: Limit span of cpu_clustergroup_mask()

From: Ionela Voinescu <ionela.voinescu@....com>

Currently the cluster identifier is not set on DT based platforms.
The reset or default value is -1 for all the CPUs. Once we assign the
cluster identifier values correctly, the cluster_sibling mask will be
populated and returned by cpu_clustergroup_mask() to contribute in the
creation of the CLS scheduling domain level, if SCHED_CLUSTER is
enabled.

To avoid topologies that will result in questionable or incorrect
scheduling domains, impose restrictions regarding the span of clusters,
as presented to scheduling domains building code: cluster_sibling should
not span more or the same CPUs as cpu_coregroup_mask().

This is needed in order to obtain a strict separation between the MC and
CLS levels, and maintain the same domains for existing platforms in
the presence of CONFIG_SCHED_CLUSTER, where the new cluster information
is redundant and irrelevant for the scheduler.

While previously the scheduling domain builder code would have removed MC
as redundant and kept CLS if SCHED_CLUSTER was enabled and the
cpu_coregroup_mask() and cpu_clustergroup_mask() spanned the same CPUs,
now CLS will be removed and MC kept.

Link: https://lore.kernel.org/r/20220621192034.3332546-18-sudeep.holla@arm.com
Cc: Darren Hart <darren@...amperecomputing.com>
Tested-by: Ionela Voinescu <ionela.voinescu@....com>
Signed-off-by: Ionela Voinescu <ionela.voinescu@....com>
Signed-off-by: Sudeep Holla <sudeep.holla@....com>
---
 drivers/base/arch_topology.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 46fa1b70b02b..277b65cf3306 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -686,6 +686,14 @@ const struct cpumask *cpu_coregroup_mask(int cpu)

 const struct cpumask *cpu_clustergroup_mask(int cpu)
 {
+	/*
+	 * Forbid cpu_clustergroup_mask() to span more or the same CPUs as
+	 * cpu_coregroup_mask().
+	 */
+	if (cpumask_subset(cpu_coregroup_mask(cpu),
+			   &cpu_topology[cpu].cluster_sibling))
+		return get_cpu_mask(cpu);
+
 	return &cpu_topology[cpu].cluster_sibling;
 }

-- 
2.36.1