linux-kernel - Re: [PATCH] sched/fair: fix incorrect calculation of capacity in scale_rt

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Date:	Wed, 30 Sep 2015 18:18:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chao Yu <chao2.yu@...sung.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: fix incorrect calculation of capacity in
 scale_rt_capacity

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@...sung.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?
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