[PATCH RFC 12/12] sched/coresched: rq->core should be set only if not previously set

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

From: Vineeth Pillai <viremana@linux.microsoft.com>

During hotplug events, smt_mask would not contain all the CPUs in a core and
this can cause reassigning of rq->core, which breaks the core-wide counters
that are needed for tracking kernel-mode entry/exits.

This patch therefore makes sure that rq->core does not change once it is set.

Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/core.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5da5b2317b21..464493f3a759 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7519,21 +7519,25 @@ int sched_cpu_starting(unsigned int cpu)
 	struct rq *rq, *core_rq = NULL;
 	int i;
 
-	for_each_cpu(i, smt_mask) {
-		rq = cpu_rq(i);
-		if (rq->core && rq->core == rq)
-			core_rq = rq;
-		init_irq_work(&rq->core_irq_work, sched_core_irq_work);
-	}
+	core_rq = cpu_rq(cpu)->core;
+
+	if (!core_rq) {
+		for_each_cpu(i, smt_mask) {
+			rq = cpu_rq(i);
+			if (rq->core && rq->core == rq)
+				core_rq = rq;
+			init_irq_work(&rq->core_irq_work, sched_core_irq_work);
+		}
 
-	if (!core_rq)
-		core_rq = cpu_rq(cpu);
+		if (!core_rq)
+			core_rq = cpu_rq(cpu);
 
-	for_each_cpu(i, smt_mask) {
-		rq = cpu_rq(i);
+		for_each_cpu(i, smt_mask) {
+			rq = cpu_rq(i);
 
-		WARN_ON_ONCE(rq->core && rq->core != core_rq);
-		rq->core = core_rq;
+			WARN_ON_ONCE(rq->core && rq->core != core_rq);
+			rq->core = core_rq;
+		}
 	}
 
 	printk("core: %d -> %d\n", cpu, cpu_of(core_rq));
-- 
2.28.0.220.ged08abb693-goog