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Date:   Fri, 14 Aug 2020 23:19:08 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     Vineeth Pillai <viremana@...ux.microsoft.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Kees Cook <keescook@...omium.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Paul Turner <pjt@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
        fweisbec@...il.com, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: [PATCH RFC 12/12] sched/coresched: rq->core should be set only if not previously set

From: Vineeth Pillai <viremana@...ux.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@...ux.microsoft.com>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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

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