linux-kernel - Re: [PATCH] sched: fair group: fix divide by zero

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Date:	Wed, 11 Jun 2008 09:15:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	mingo@...e.hu,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: fair group: fix divide by zero

On Wed, 2008-06-11 at 15:12 +0800, Lai Jiangshan wrote:
> I found a bug which can be reproduced by this way:(linux-2.6.26-rc5, x86-64)
> (use 2^32, 2^33, ...., 2^63 as shares value)

I think the same thing to do is limit the shares value to something
smaller instead of using an even more expensive divide.

> # mkdir /dev/cpuctl
> # mount -t cgroup -o cpu cpuctl /dev/cpuctl
> # cd /dev/cpuctl
> # mkdir sub
> # echo 0x8000000000000000 > sub/cpu.shares
> # echo $$ > sub/tasks
> oops here! divide by zero.
> 
> This is because do_div() expects the 2th parameter to be 32 bits,
> but unsigned long is 64 bits in x86_64.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 08ae848..d3005b4 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -368,7 +368,7 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  		cfs_rq = cfs_rq_of(se);
>  
>  		slice *= se->load.weight;
> -		do_div(slice, cfs_rq->load.weight);
> +		slice = div64_u64(slice, cfs_rq->load.weight);
>  	}
>  
> 
> @@ -399,7 +399,7 @@ static u64 sched_vslice_add(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  			weight += se->load.weight;
>  
>  		vslice *= NICE_0_LOAD;
> -		do_div(vslice, weight);
> +		vslice = div64_u64(vslice, weight);
>  	}
>  
>  	return vslice;
> 
> 

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