[PATCH_V2] sched/fair: updates weight of cfs_rq before update_cfs_group() in enqueue_entity()

[Gaowei.Pu]

From: pugaowei <pugaowei@xxxxxxxx>

we should update the weight of cfs_rq before update_cfs_group().
Ensure that we can get accurate shares of the cfs_rq when its
weights changes. we can find this work was done correctly in
dequeue_entity(). so fix it.

patch_V1 :
https://lore.kernel.org/lkml/20240531030833.3375-1-pugaowei@xxxxxxxx/T/#u
trigger a warnning below because of the changing order of
account_entity_enqueue().

[ 0.400603][ T0] ? __warn (kernel/panic.c:693)

[ 0.400603][ T0] ? place_entity (kernel/sched/fair.c:5256 (discriminator 1))

[ 0.400603][ T0] ? report_bug (lib/bug.c:180 lib/bug.c:219)

[ 0.400603][ T0] ? handle_bug (arch/x86/kernel/traps.c:239)

[ 0.400603][ T0] ? exc_invalid_op (arch/x86/kernel/traps.c:260 (discriminator 1))

[ 0.400603][ T0] ? asm_exc_invalid_op (arch/x86/include/asm/idtentry.h:621)

[ 0.400603][ T0] ? place_entity (kernel/sched/fair.c:5256 (discriminator 1))

[ 0.400603][ T0] ? place_entity (kernel/sched/fair.c:5182)

[ 0.400603][ T0] enqueue_entity (kernel/sched/fair.c:5328)

[ 0.400603][ T0] enqueue_task_fair (kernel/sched/fair.c:6785)

V2 fix the warnning and keep the lag without inflating it when it is
the first sched_entity queued on the cfs_rq.

Signed-off-by: pugaowei <pugaowei@xxxxxxxx>
---
 kernel/sched/fair.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8a5b1ae0aa55..2fb1fbcfdda3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5190,12 +5190,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	 *
 	 * EEVDF: placement strategy #1 / #2
 	 */
-	if (sched_feat(PLACE_LAG) && cfs_rq->nr_running) {
+	if (sched_feat(PLACE_LAG)) {
 		struct sched_entity *curr = cfs_rq->curr;
-		unsigned long load;
+		unsigned long load, se_load;
 
 		lag = se->vlag;
-
+		se_load = scale_load_down(se->load.weight);
 		/*
 		 * If we want to place a task and preserve lag, we have to
 		 * consider the effect of the new entity on the weighted
@@ -5252,9 +5252,13 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 		if (curr && curr->on_rq)
 			load += scale_load_down(curr->load.weight);
 
-		lag *= load + scale_load_down(se->load.weight);
-		if (WARN_ON_ONCE(!load))
-			load = 1;
+		lag *= load + se_load;
+		/*
+		 * we just need to keep the lag whithout inflating it when the se is
+		 * the first sched_entity queued on cfs_rq.
+		 */
+		if (!load)
+			load = se_load;
 		lag = div_s64(lag, load);
 	}
 
@@ -5304,6 +5308,12 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	 */
 	update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH);
 	se_update_runnable(se);
+
+	/*
+	 * we should update the weight of cfs_rq before update_cfs_group.
+	 * Ensure we can get accurate shares of the cfs_rq when its weights changes.
+	 */
+	account_entity_enqueue(cfs_rq, se);
 	/*
 	 * XXX update_load_avg() above will have attached us to the pelt sum;
 	 * but update_cfs_group() here will re-adjust the weight and have to
@@ -5318,8 +5328,6 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	if (!curr)
 		place_entity(cfs_rq, se, flags);
 
-	account_entity_enqueue(cfs_rq, se);
-
 	/* Entity has migrated, no longer consider this task hot */
 	if (flags & ENQUEUE_MIGRATED)
 		se->exec_start = 0;
-- 
2.17.1