[PATCH] sched/fair: fix incorrect calculation of capacity in scale_rt_capacity

[PATCH] sched/fair: fix incorrect calculation of capacity in scale_rt_capacity

[Chao Yu]

div_u64 can only handle 32-bits divisor, if our divisor is with type of
64-bits, we should use div64_u64 instead, otherwise value of divisor will
be cast to 32-bits, resulting in wrong calculation.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9176f7c..ee268ef 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6086,7 +6086,7 @@ static unsigned long scale_rt_capacity(int cpu)
 
 	total = sched_avg_period() + delta;
 
-	used = div_u64(avg, total);
+	used = div64_u64(avg, total);
 
 	if (likely(used < SCHED_CAPACITY_SCALE))
 		return SCHED_CAPACITY_SCALE - used;
-- 
2.5.2

Re: [PATCH] sched/fair: fix incorrect calculation of capacity in scale_rt_capacity

[Peter Zijlstra]

On Wed, Sep 23, 2015 at 02:15:51PM +0800, Chao Yu wrote:
> div_u64 can only handle 32-bits divisor, if our divisor is with type of
> 64-bits, we should use div64_u64 instead, otherwise value of divisor will
> be cast to 32-bits, resulting in wrong calculation.
> 
> Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9176f7c..ee268ef 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6086,7 +6086,7 @@ static unsigned long scale_rt_capacity(int cpu)
>  
>  	total = sched_avg_period() + delta;
>  
> -	used = div_u64(avg, total);
> +	used = div64_u64(avg, total);

total here should not exceed 2*sched_avg_period() 'much', with the
defaults this would end up being around 1e9.

So unless you prod at sysctl_sched_time_avg it should all work out fine
with the 32bit divisor.

Have you seen problems here?

RE: [PATCH] sched/fair: fix incorrect calculation of capacity in scale_rt_capacity

[Chao Yu]

Ping.

> -----Original Message-----
> From: Chao Yu [mailto:chao2.yu@samsung.com]
> Sent: Wednesday, September 23, 2015 2:16 PM
> To: 'mingo@redhat.com'; 'peterz@infradead.org'
> Cc: 'linux-kernel@vger.kernel.org'
> Subject: [PATCH] sched/fair: fix incorrect calculation of capacity in scale_rt_capacity
> 
> div_u64 can only handle 32-bits divisor, if our divisor is with type of
> 64-bits, we should use div64_u64 instead, otherwise value of divisor will
> be cast to 32-bits, resulting in wrong calculation.
> 
> Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9176f7c..ee268ef 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6086,7 +6086,7 @@ static unsigned long scale_rt_capacity(int cpu)
> 
>  	total = sched_avg_period() + delta;
> 
> -	used = div_u64(avg, total);
> +	used = div64_u64(avg, total);
> 
>  	if (likely(used < SCHED_CAPACITY_SCALE))
>  		return SCHED_CAPACITY_SCALE - used;
> --
> 2.5.2