sched: support dl task migration during cpu hotplug and other fixes

Dl class will refuse the bandwidth being set to some value smaller than the currently allocated bandwidth in any of the root_domains through sched_rt_runtime_us and sched_rt_period_us. RT runtime will be set according to sched_rt_runtime_us before dl class verify if the new bandwidth is suitable in the case of !CONFIG_RT_GROUP_SCHED.

However, rt runtime will be corrupt if dl refuse the new bandwidth since there is no undo to reset the rt runtime to the old value.

This patch fix it by verifying new bandwidth for deadline in advance.

Cc Juri Lelli <[email protected]>

Signed-off-by Wanpeng Li <[email protected]>

---
v1 -> v2:
* move sched_dl_global_constraints before sched_rt_global_constraints, and change the name of the former to sched_dl_global_validate().

kernel/sched/core.c | 6 +++---

1 file changed, 3 insertions(+), 3 deletions(-)

kernel/sched/core.c +3 -3

index 603c462..981f075 100644
@@ -7808,7 +7808,7 @@ static int sched_rt_global_constraints(void)
 }
 #endif /* CONFIG_RT_GROUP_SCHED */
 
-static int sched_dl_global_constraints(void)
+static int sched_dl_global_validate(void)
 {
 	u64 runtime = global_rt_runtime();
 	u64 period = global_rt_period();

@@ -7909,11 +7909,11 @@ int sched_rt_handler(struct ctl_table *table, int write,
 		if (ret)
 			goto undo;
 
-		ret = sched_rt_global_constraints();
+		ret = sched_dl_global_validate();
 		if (ret)
 			goto undo;
 
-		ret = sched_dl_global_constraints();
+		ret = sched_rt_global_constraints();
 		if (ret)
 			goto undo;
 

Signature · 1 line

1.9.1

The overload indicator is used for knowing when we can totally avoid load balancing to a cpu that is about to go idle. We can avoid load balancing when no cpu has cfs task and both rt and deadline have push/pull mechanism to do their own balancing.

However, rq->nr_running on behalf of the total number of each class tasks on the cpu, do idle balance when remaining tasks are all non-CFS tasks does not make any sense.

This patch fix it by idle balance when there are still other CFS tasks in the rq's root domain.

kernel/sched/sched.h +0 -0

index 31f1e4d..f7dd978 100644

Quoted 5 lines · Wanpeng Li

The overload indicator is used for knowing when we can totally avoid load balancing to a cpu that is about to go idle. We can avoid load balancing when no cpu has cfs task and both rt and deadline have push/pull mechanism to do their own balancing.

Quoted 7 lines

However, rq->nr_running on behalf of the total number of each class tasks on the cpu, do idle balance when remaining tasks are all non-CFS tasks does not make any sense.

This patch fix it by idle balance when there are still other CFS tasks in the rq's root domain.

Please always try and Cc the people who touched that code last; for the idle_balance bits commit 4486edd12b5a ("sched/fair: Implement fast idling of CPUs when the system is partially loaded") gives a fair clue as to who that would be.

Quoted 6 lines

kernel/sched/sched.h +0 -0

index 31f1e4d..f7dd978 100644
@@ -1269,7 +1269,8 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
 

kernel/sched/sched.h static inline void add_nr_running(struct rq *rq, unsigned count) · line 1276

Quoted 8 lines

kernel/sched/sched.h +2 -1

 	rq->nr_running = prev_nr + count;
 
-	if (prev_nr < 2 && rq->nr_running >= 2) {
+	if (prev_nr < 2 && rq->nr_running >= 2 &&
+		rq->cfs.h_nr_running > 0) {
 #ifdef CONFIG_SMP
 		if (!rq->rd->overload)
 			rq->rd->overload = true;

Here 3882ec643997 ("nohz: Use IPI implicit full barrier against rq->nr_running r/w") might be a clue.

Also, this is wrong, it breaks NOHZ_FULL.

Quoted 25 lines · Peter Zijlstra

Quoted 5 lines · Wanpeng Li

The overload indicator is used for knowing when we can totally avoid load balancing to a cpu that is about to go idle. We can avoid load balancing when no cpu has cfs task and both rt and deadline have push/pull mechanism to do their own balancing.

Quoted 7 lines

However, rq->nr_running on behalf of the total number of each class tasks on the cpu, do idle balance when remaining tasks are all non-CFS tasks does not make any sense.

This patch fix it by idle balance when there are still other CFS tasks in the rq's root domain.

Please always try and Cc the people who touched that code last; for the idle_balance bits commit 4486edd12b5a ("sched/fair: Implement fast idling of CPUs when the system is partially loaded") gives a fair clue as to who that would be.

Quoted 6 lines

kernel/sched/sched.h +0 -0

index 31f1e4d..f7dd978 100644
@@ -1269,7 +1269,8 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
 

Quoted 9 lines

kernel/sched/sched.h static inline void add_nr_running(struct rq *rq, unsigned count) · line 1276

Quoted 8 lines

kernel/sched/sched.h +2 -1

 	rq->nr_running = prev_nr + count;
 
-	if (prev_nr < 2 && rq->nr_running >= 2) {
+	if (prev_nr < 2 && rq->nr_running >= 2 &&
+		rq->cfs.h_nr_running > 0) {
 #ifdef CONFIG_SMP
 		if (!rq->rd->overload)
 			rq->rd->overload = true;

After this change, you will not start cfs load balance if you have one cfs task and a few other non-cfs tasks on a run-queue. In this case if there's a less loaded run queue available, the cfs scheduler would not move the cfs task there. So you will be forcing the deadline and rt scheduler to move their tasks away. In this case, a cfs task will behave like a "higher" priority task than the rt and deadline tasks in the sense that it forces the other classes of tasks to be moved to accommodate cfs tasks. I don't think this is the right behavior.

Also, add_nr_running could add more than one cfs tasks. So if there are more than one cfs tasks being added, you should load balance and that need to be checked in add_nr_running if you make the change there.

Tim

Quoted 4 lines

Here 3882ec643997 ("nohz: Use IPI implicit full barrier against rq->nr_running r/w") might be a clue.

Also, this is wrong, it breaks NOHZ_FULL.

Quoted 5 lines · Wanpeng Li

The overload indicator is used for knowing when we can totally avoid load balancing to a cpu that is about to go idle. We can avoid load balancing when no cpu has cfs task and both rt and deadline have push/pull mechanism to do their own balancing.

Quoted 7 lines

However, rq->nr_running on behalf of the total number of each class tasks on the cpu, do idle balance when remaining tasks are all non-CFS tasks does not make any sense.

This patch fix it by idle balance when there are still other CFS tasks in the rq's root domain.

Please always try and Cc the people who touched that code last; for the idle_balance bits commit 4486edd12b5a ("sched/fair: Implement fast idling of CPUs when the system is partially loaded") gives a fair clue as to who that would be.

Quoted 5 lines

kernel/sched/sched.h +0 -0

index 31f1e4d..f7dd978 100644

kernel/sched/sched.h static inline void add_nr_running(struct rq *rq, unsigned count) · line 1276

Quoted 9 lines

kernel/sched/sched.h +2 -1

@@ -1269,7 +1269,8 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
 
 	rq->nr_running = prev_nr + count;
 
-	if (prev_nr < 2 && rq->nr_running >= 2) {
+	if (prev_nr < 2 && rq->nr_running >= 2 &&
+		rq->cfs.h_nr_running > 0) {
 #ifdef CONFIG_SMP
 		if (!rq->rd->overload)
 			rq->rd->overload = true;

Here 3882ec643997 ("nohz: Use IPI implicit full barrier against rq->nr_running r/w") might be a clue.

Also, this is wrong, it breaks NOHZ_FULL.

Quoted 7 lines

kernel/sched/sched.h static inline void add_nr_running(struct rq *rq, unsigned count) · line 1276

Quoted 2 lines

kernel/sched/sched.h +0 -0

 		if (!rq->rd->overload)
 			rq->rd->overload = true;

Here 3882ec643997 ("nohz: Use IPI implicit full barrier against rq->nr_running r/w") might be a clue.

Also, this is wrong, it breaks NOHZ_FULL.

Got it. If the modification in update_sg_lb_stats of my patch is still corrrect, if so I can send out a new version.

Regards,
Wanpeng Li

Quoted 6 lines · Wanpeng Li

Quoted 6 lines

kernel/sched/sched.h static inline void add_nr_running(struct rq *rq, unsigned count) · line 1276

Quoted 2 lines

kernel/sched/sched.h +0 -0

 		if (!rq->rd->overload)
 			rq->rd->overload = true;

Here 3882ec643997 ("nohz: Use IPI implicit full barrier against rq->nr_running r/w") might be a clue.

Quoted 4 lines

Quoted 1 line

Also, this is wrong, it breaks NOHZ_FULL.

Got it. If the modification in update_sg_lb_stats of my patch is still corrrect, if so I can send out a new version.

Ping,

Quoted 3 lines

Regards,
Wanpeng Li

I collect recent send out patches to one patchset in order to get review easily.

For 7/7 I will remove rd->span related modification if Pang's effort merged.
https://lkml.org/lkml/2014/11/17/443

Wanpeng Li (7):
sched/deadline: fix start high-res preemption tick for a non-leftmost task sched/deadline: fix rt runtime corrupt when dl refuse a smaller bandwidth sched/deadline: fix dl entity is still mark yield after replenishing sched/deadline: reduce overhead if there are no scheduling parameters changed sched/fair: fix idle balance when remaining tasks are all non-CFS tasks
sched: fix start hrtick for short schedule slices on UP sched/deadline: support dl task migration during cpu hotplug

kernel/sched/core.c     | 27 +++++++++++++++++----
kernel/sched/deadline.c | 62 +++++++++++++++++++++++++++++++++++++++++++++----
kernel/sched/fair.c     |  2 +-
kernel/sched/sched.h    |  3 ++-
4 files changed, 84 insertions(+), 10 deletions(-)

Signature · 1 line

1.9.1