diff mbox series

[1/1,v3] sched/pelt: Fix the attach_entity_load_avg calculate method

Message ID 20220414090229.342-1-kuyo.chang@mediatek.com (mailing list archive)
State New, archived
Headers show
Series [1/1,v3] sched/pelt: Fix the attach_entity_load_avg calculate method | expand

Commit Message

Kuyo Chang (張建文) April 14, 2022, 9:02 a.m. UTC 
From: kuyo chang <kuyo.chang@mediatek.com>

I meet the warning message at cfs_rq_is_decayed at below code.

SCHED_WARN_ON(cfs_rq->avg.load_avg ||
		    cfs_rq->avg.util_avg ||
		    cfs_rq->avg.runnable_avg)

Following is the calltrace.

Call trace:
__update_blocked_fair
update_blocked_averages
newidle_balance
pick_next_task_fair
__schedule
schedule
pipe_read
vfs_read
ksys_read

After code analyzing and some debug messages, I found it exits a corner
case at attach_entity_load_avg which will cause load_sum is null but
load_avg is not.
Consider se_weight is 88761 according by sched_prio_to_weight table.
And assume the get_pelt_divider() is 47742, se->avg.load_avg is 1.
By the calculating for se->avg.load_sum as following will become zero
as following.
se->avg.load_sum =
	div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
se->avg.load_sum = 1*47742/88761 = 0.

After enqueue_load_avg code as below.
cfs_rq->avg.load_avg += se->avg.load_avg;
cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;

Then the load_sum for cfs_rq will be 1 while the load_sum for cfs_rq is 0.
So it will hit the warning message.

In order to fix the corner case, make sure the se->load_avg|sum is correct
before enqueue_load_avg.

After long time testing, the kernel warning was gone and the system runs
as well as before.

Fixes: f207934fb79d ("sched/fair: Align PELT windows between cfs_rq and its se")
Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
---

 v1->v2:
 (1)Thanks for suggestion from Peter Zijlstra & Vincent Guittot.
 (2)By suggestion from Vincent Guittot,
 rework the se->load_sum calculation method for fix the corner case,
 make sure the se->load_avg|sum is correct before enqueue_load_avg.
 (3)Rework changlog.

 v2->v3:
 (1)Rename Subject. 
 (1)Add fix tag.


 kernel/sched/fair.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

Comments

Peter Zijlstra April 14, 2022, 2:44 p.m. UTC | #1 
I've taken the liberty of carrying over the tags from v2 and reworked
the Changelog a little.

---
Subject: sched/pelt: Fix attach_entity_load_avg() corner case
From: kuyo chang <kuyo.chang@mediatek.com>
Date: Thu, 14 Apr 2022 17:02:20 +0800

From: kuyo chang <kuyo.chang@mediatek.com>

The warning in cfs_rq_is_decayed() triggered:

    SCHED_WARN_ON(cfs_rq->avg.load_avg ||
		  cfs_rq->avg.util_avg ||
		  cfs_rq->avg.runnable_avg)

There exists a corner case in attach_entity_load_avg() which will
cause load_sum to be zero while load_avg will not be.

Consider se_weight is 88761 as per the sched_prio_to_weight[] table.
Further assume the get_pelt_divider() is 47742, this gives:
se->avg.load_avg is 1.

However, calculating load_sum results in 0:

  se->avg.load_sum = div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
  se->avg.load_sum = 1*47742/88761 = 0.

Then enqueue_load_avg() adds this to the cfs_rq totals:

  cfs_rq->avg.load_avg += se->avg.load_avg;
  cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;

Resulting in load_avg being 1 with load_sum is 0, which will trigger
the WARN.

Fixes: f207934fb79d ("sched/fair: Align PELT windows between cfs_rq and its se")
Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
[peterz: massage changelog]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lkml.kernel.org/r/20220414090229.342-1-kuyo.chang@mediatek.com
---
 kernel/sched/fair.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3829,11 +3829,11 @@ static void attach_entity_load_avg(struc
 
 	se->avg.runnable_sum = se->avg.runnable_avg * divider;
 
-	se->avg.load_sum = divider;
-	if (se_weight(se)) {
-		se->avg.load_sum =
-			div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
-	}
+	se->avg.load_sum = se->avg.load_avg * divider;
+	if (se_weight(se) < se->avg.load_sum)
+		se->avg.load_sum = div_u64(se->avg.load_sum, se_weight(se));
+	else
+		se->avg.load_sum = 1;
 
 	enqueue_load_avg(cfs_rq, se);
 	cfs_rq->avg.util_avg += se->avg.util_avg;
Kuyo Chang (張建文) April 15, 2022, 3:56 a.m. UTC | #2 
On Thu, 2022-04-14 at 16:44 +0200, Peter Zijlstra wrote:
> I've taken the liberty of carrying over the tags from v2 and reworked
> the Changelog a little.

Thank you for all your assistance.

> ---
> Subject: sched/pelt: Fix attach_entity_load_avg() corner case
> From: kuyo chang <kuyo.chang@mediatek.com>
> Date: Thu, 14 Apr 2022 17:02:20 +0800
> 
> From: kuyo chang <kuyo.chang@mediatek.com>
> 
> The warning in cfs_rq_is_decayed() triggered:
> 
>     SCHED_WARN_ON(cfs_rq->avg.load_avg ||
> 		  cfs_rq->avg.util_avg ||
> 		  cfs_rq->avg.runnable_avg)
> 
> There exists a corner case in attach_entity_load_avg() which will
> cause load_sum to be zero while load_avg will not be.
> 
> Consider se_weight is 88761 as per the sched_prio_to_weight[] table.
> Further assume the get_pelt_divider() is 47742, this gives:
> se->avg.load_avg is 1.
> 
> However, calculating load_sum results in 0:
> 
>   se->avg.load_sum = div_u64(se->avg.load_avg * se->avg.load_sum,
> se_weight(se));
>   se->avg.load_sum = 1*47742/88761 = 0.
> 
> Then enqueue_load_avg() adds this to the cfs_rq totals:
> 
>   cfs_rq->avg.load_avg += se->avg.load_avg;
>   cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
> 
> Resulting in load_avg being 1 with load_sum is 0, which will trigger
> the WARN.
> 
> Fixes: f207934fb79d ("sched/fair: Align PELT windows between cfs_rq
> and its se")
> Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> [peterz: massage changelog]
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Link: 
> https://urldefense.com/v3/__https://lkml.kernel.org/r/20220414090229.342-1-kuyo.chang@mediatek.com__;!!CTRNKA9wMg0ARbw!35Im02xxIuUZdLYpPng37Yk7oVNJVJ1tfbu4XRzlq-6VhH3K29Por0gJCFlslT_CMgA$
>  
> ---
>  kernel/sched/fair.c |   10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3829,11 +3829,11 @@ static void attach_entity_load_avg(struc
>  
>  	se->avg.runnable_sum = se->avg.runnable_avg * divider;
>  
> -	se->avg.load_sum = divider;
> -	if (se_weight(se)) {
> -		se->avg.load_sum =
> -			div_u64(se->avg.load_avg * se->avg.load_sum,
> se_weight(se));
> -	}
> +	se->avg.load_sum = se->avg.load_avg * divider;
> +	if (se_weight(se) < se->avg.load_sum)
> +		se->avg.load_sum = div_u64(se->avg.load_sum,
> se_weight(se));
> +	else
> +		se->avg.load_sum = 1;
>  
>  	enqueue_load_avg(cfs_rq, se);
>  	cfs_rq->avg.util_avg += se->avg.util_avg;
diff mbox series

Patch

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d4bd299d67ab..159274482c4e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3829,10 +3829,12 @@  static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
 
 	se->avg.runnable_sum = se->avg.runnable_avg * divider;
 
-	se->avg.load_sum = divider;
-	if (se_weight(se)) {
+	se->avg.load_sum = se->avg.load_avg * divider;
+	if (se_weight(se) < se->avg.load_sum) {
 		se->avg.load_sum =
-			div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
+			div_u64(se->avg.load_sum, se_weight(se));
+	} else {
+		se->avg.load_sum = 1;
 	}
 
 	enqueue_load_avg(cfs_rq, se);