[PATCH 0/2] sched/eevdf: Minor fixes for reweight_entity()

[PATCH 0/2] sched/eevdf: Minor fixes for reweight_entity()

[Tianchen Ding]

According to the discussion in [1], fix a calculation bug in
reweight_entity().

[1] https://lore.kernel.org/all/59585184-d13d-46e0-8d68-42838e97a702@bytedance.com/

Tianchen Ding (2):
  sched/eevdf: Always update V if se->on_rq when reweighting
  sched/eevdf: Fix miscalculation in reweight_entity() when se is not
    curr

 kernel/sched/fair.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

-- 
2.39.3

[PATCH 1/2] sched/eevdf: Always update V if se->on_rq when reweighting

[Tianchen Ding]

reweight_eevdf() needs the latest V to do accurate calculation for new
ve and vd. So update V unconditionally when se is runnable.

Suggested-by: Abel Wu <wuyun.abel@bytedance.com>
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
---
 kernel/sched/fair.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 03be0d1330a6..5551ce2af73e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3790,9 +3790,8 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 
 	if (se->on_rq) {
 		/* commit outstanding execution time */
-		if (curr)
-			update_curr(cfs_rq);
-		else
+		update_curr(cfs_rq);
+		if (!curr)
 			__dequeue_entity(cfs_rq, se);
 		update_load_sub(&cfs_rq->load, se->load.weight);
 	}
-- 
2.39.3

Re: [PATCH 1/2] sched/eevdf: Always update V if se->on_rq when reweighting

[Abel Wu]

On 3/4/24 11:00 AM, Tianchen Ding Wrote:
> reweight_eevdf() needs the latest V to do accurate calculation for new
> ve and vd. So update V unconditionally when se is runnable.

As this should come along with commit eab03c23c2a1, I think it would
be better add a Fixes tag?

> 
> Suggested-by: Abel Wu <wuyun.abel@bytedance.com>
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
> ---
>   kernel/sched/fair.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 03be0d1330a6..5551ce2af73e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3790,9 +3790,8 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>   
>   	if (se->on_rq) {
>   		/* commit outstanding execution time */
> -		if (curr)
> -			update_curr(cfs_rq);
> -		else
> +		update_curr(cfs_rq);
> +		if (!curr)
>   			__dequeue_entity(cfs_rq, se);
>   		update_load_sub(&cfs_rq->load, se->load.weight);
>   	}

Re: [PATCH 1/2] sched/eevdf: Always update V if se->on_rq when reweighting

[Tianchen Ding]

On 2024/3/4 20:06, Abel Wu wrote:
> On 3/4/24 11:00 AM, Tianchen Ding Wrote:
>> reweight_eevdf() needs the latest V to do accurate calculation for new
>> ve and vd. So update V unconditionally when se is runnable.
> 
> As this should come along with commit eab03c23c2a1, I think it would
> be better add a Fixes tag?
> 

I'm not sure. In fact avg_vruntime_sub() in reweight_entity() was first 
introduced by af4cf40470c2, and at that time the algorithm about reweight was 
not completed...

Re: Re: [PATCH 1/2] sched/eevdf: Always update V if se->on_rq when reweighting

[Abel Wu]

On 3/5/24 10:13 AM, Tianchen Ding Wrote:
> On 2024/3/4 20:06, Abel Wu wrote:
>> On 3/4/24 11:00 AM, Tianchen Ding Wrote:
>>> reweight_eevdf() needs the latest V to do accurate calculation for new
>>> ve and vd. So update V unconditionally when se is runnable.
>>
>> As this should come along with commit eab03c23c2a1, I think it would
>> be better add a Fixes tag?
>>
> 
> I'm not sure. In fact avg_vruntime_sub() in reweight_entity() was first introduced by af4cf40470c2, and at that time the algorithm about reweight was not completed...
> 

It is because before commit eab03c23c2a1, reweight didn't rely on V.

[PATCH 2/2] sched/eevdf: Fix miscalculation in reweight_entity() when se is not curr

[Tianchen Ding]

reweight_eevdf() only keeps V unchanged inside itself. When se !=
cfs_rq->curr, it would be dequeued from rb tree first. So that V is
changed and the result is wrong. Pass the original V to reweight_eevdf()
to fix this issue.

Fixes: eab03c23c2a1 ("sched/eevdf: Fix vruntime adjustment on reweight")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
---
 kernel/sched/fair.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5551ce2af73e..091a1a750638 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3676,11 +3676,10 @@ static inline void
 dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
 #endif
 
-static void reweight_eevdf(struct cfs_rq *cfs_rq, struct sched_entity *se,
+static void reweight_eevdf(struct sched_entity *se, u64 avruntime,
 			   unsigned long weight)
 {
 	unsigned long old_weight = se->load.weight;
-	u64 avruntime = avg_vruntime(cfs_rq);
 	s64 vlag, vslice;
 
 	/*
@@ -3787,10 +3786,12 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 			    unsigned long weight)
 {
 	bool curr = cfs_rq->curr == se;
+	u64 avruntime;
 
 	if (se->on_rq) {
 		/* commit outstanding execution time */
 		update_curr(cfs_rq);
+		avruntime = avg_vruntime(cfs_rq);
 		if (!curr)
 			__dequeue_entity(cfs_rq, se);
 		update_load_sub(&cfs_rq->load, se->load.weight);
@@ -3804,7 +3805,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 		 */
 		se->vlag = div_s64(se->vlag * se->load.weight, weight);
 	} else {
-		reweight_eevdf(cfs_rq, se, weight);
+		reweight_eevdf(se, avruntime, weight);
 	}
 
 	update_load_set(&se->load, weight);
-- 
2.39.3

Re: [PATCH 2/2] sched/eevdf: Fix miscalculation in reweight_entity() when se is not curr

[Abel Wu]

On 3/4/24 11:00 AM, Tianchen Ding Wrote:
> reweight_eevdf() only keeps V unchanged inside itself. When se !=
> cfs_rq->curr, it would be dequeued from rb tree first. So that V is
> changed and the result is wrong. Pass the original V to reweight_eevdf()
> to fix this issue.
> 
> Fixes: eab03c23c2a1 ("sched/eevdf: Fix vruntime adjustment on reweight")
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
> ---
>   kernel/sched/fair.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5551ce2af73e..091a1a750638 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3676,11 +3676,10 @@ static inline void
>   dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
>   #endif
>   
> -static void reweight_eevdf(struct cfs_rq *cfs_rq, struct sched_entity *se,
> +static void reweight_eevdf(struct sched_entity *se, u64 avruntime,
>   			   unsigned long weight)
>   {
>   	unsigned long old_weight = se->load.weight;
> -	u64 avruntime = avg_vruntime(cfs_rq);
>   	s64 vlag, vslice;
>   
>   	/*
> @@ -3787,10 +3786,12 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>   			    unsigned long weight)
>   {
>   	bool curr = cfs_rq->curr == se;
> +	u64 avruntime;
>   
>   	if (se->on_rq) {
>   		/* commit outstanding execution time */
>   		update_curr(cfs_rq);
> +		avruntime = avg_vruntime(cfs_rq);
>   		if (!curr)
>   			__dequeue_entity(cfs_rq, se);
>   		update_load_sub(&cfs_rq->load, se->load.weight);
> @@ -3804,7 +3805,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
>   		 */
>   		se->vlag = div_s64(se->vlag * se->load.weight, weight);
>   	} else {
> -		reweight_eevdf(cfs_rq, se, weight);
> +		reweight_eevdf(se, avruntime, weight);
>   	}
>   
>   	update_load_set(&se->load, weight);

Reviewed-by: Abel Wu <wuyun.abel@bytedance.com>