[PATCH 1/2] sched/fair: Don't rely on ->exec_start for migration

[Tim Chen]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

From: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Currently migrate_task_rq_fair() (ab)uses se->exec_start to make
task_hot() fail. In order to preserve ->exec_start, add a ->migrated
flag to sched_entity.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 include/linux/sched.h | 1 +
 kernel/sched/fair.c   | 6 +++++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 75ba8aa60248..0edf16b4d40a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -541,6 +541,7 @@ struct sched_entity {
 	struct rb_node			run_node;
 	struct list_head		group_node;
 	unsigned int			on_rq;
+	unsigned int			migrated;
 
 	u64				exec_start;
 	u64				sum_exec_runtime;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5146163bfabb..2498e97804fd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1004,6 +1004,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	/*
 	 * We are starting a new run period:
 	 */
+	se->migrated = 0;
 	se->exec_start = rq_clock_task(rq_of(cfs_rq));
 }
 
@@ -6979,7 +6980,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
 	p->se.avg.last_update_time = 0;
 
 	/* We have migrated, no longer consider this task hot */
-	p->se.exec_start = 0;
+	p->se.migrated = 1;
 
 	update_scan_period(p, new_cpu);
 }
@@ -7665,6 +7666,9 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
 	if (sysctl_sched_migration_cost == 0)
 		return 0;
 
+	if (p->se.migrated)
+		return 0;
+
 	delta = rq_clock_task(env->src_rq) - p->se.exec_start;
 
 	return delta < (s64)sysctl_sched_migration_cost;
-- 
2.32.0