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

[Joel Fernandes (Google)]

From: Vineeth Pillai <virem...@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 <virem...@linux.microsoft.com>
Signed-off-by: Joel Fernandes (Google) <j...@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