[PATCH] sched: Fix tg->load when offlining a CPU

[Vincent Guittot]

When a CPU taken offline, the contribution of its cfs_rqs to task_groups'
load may remain and impact the calculation of the share of the online
CPUs.
Clear the contribution of an offlining CPU to task groups' load and skip
its contribution while it is inactive.

Reported-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
 kernel/sched/fair.c | 50 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 07200b4751b3..07edb7dce671 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4094,6 +4094,10 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
 	if (cfs_rq->tg == &root_task_group)
 		return;
 
+	/* rq has been offline and doesn't contribute anymore to the share */
+	if (!cpu_active(cpu_of(rq_of(cfs_rq))))
+		return;
+
 	/*
 	 * For migration heavy workloads, access to tg->load_avg can be
 	 * unbound. Limit the update rate to at most once per ms.
@@ -4110,6 +4114,49 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
 	}
 }
 
+static inline void clear_tg_load_avg(struct cfs_rq *cfs_rq)
+{
+	long delta;
+	u64 now;
+
+	/*
+	 * No need to update load_avg for root_task_group as it is not used.
+	 */
+	if (cfs_rq->tg == &root_task_group)
+		return;
+
+	now = sched_clock_cpu(cpu_of(rq_of(cfs_rq)));
+	delta = 0 - cfs_rq->tg_load_avg_contrib;
+	atomic_long_add(delta, &cfs_rq->tg->load_avg);
+	cfs_rq->tg_load_avg_contrib = 0;
+	cfs_rq->last_update_tg_load_avg = now;
+}
+
+/* cpu offline callback */
+static void __maybe_unused clear_tg_offline_cfs_rqs(struct rq *rq)
+{
+	struct task_group *tg;
+
+	lockdep_assert_rq_held(rq);
+
+	/*
+	 * The rq clock has already been updated in the
+	 * set_rq_offline(), so we should skip updating
+	 * the rq clock again in unthrottle_cfs_rq().
+	 */
+	rq_clock_start_loop_update(rq);
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(tg, &task_groups, list) {
+		struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
+
+		clear_tg_load_avg(cfs_rq);
+	}
+	rcu_read_unlock();
+
+	rq_clock_stop_loop_update(rq);
+}
+
 /*
  * Called within set_task_rq() right before setting a task's CPU. The
  * caller only guarantees p->pi_lock is held; no other assumptions,
@@ -12423,6 +12470,9 @@ static void rq_offline_fair(struct rq *rq)
 
 	/* Ensure any throttled groups are reachable by pick_next_task */
 	unthrottle_offline_cfs_rqs(rq);
+
+	/* Ensure that we remove rq contribution to group share */
+	clear_tg_offline_cfs_rqs(rq);
 }
 
 #endif /* CONFIG_SMP */
-- 
2.34.1