Re: [PATCH] sched: fair-group: fix a Div0 error of the fair groupscheduler

From: Peter Zijlstra
Date: Mon Apr 28 2008 - 04:34:43 EST


On Mon, 2008-04-28 at 12:54 +0800, Miao Xie wrote:
> When I echoed 0 into the "cpu.shares" file, a Div0 error occured.
>
> We found it is caused by the following calling.
>
> sched_group_set_shares(tg, shares)
> set_se_shares(tg->se[i], shares/nr_cpu_ids)
> __set_se_shares(se, shares)
> div64_64((1ULL<<32), shares)
>
> When the echoed value was less than the number of processores, the result of the
> sentence "shares/nr_cpu_ids" was 0, and then the system called div64() to divide
> the result, the Div0 error occured.
>
> It is unnecessary that the shares value is divided by nr_cpu_ids, I think.
> Because in the function __update_group_shares_cpu() and init_tg_cfs_entry(),
> the shares value isn't divided by nr_cpu_ids when setting shares of the sched
> entity.
>
> This patch fixes this bug. And echoing ULONG_MAX value into cpu.shares also
> causes Div0 error, so we set a macro MAX_SHARES to limit the max value of
> shares.
>
> Signed-off-by: Miao Xie <miaox@xxxxxxxxxxxxxx>

Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>

> ---
> kernel/sched.c | 17 +++++++++++------
> 1 files changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 740fb40..aa1bb81 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -318,7 +318,13 @@ static DEFINE_MUTEX(doms_cur_mutex);
> # define INIT_TASK_GROUP_LOAD NICE_0_LOAD
> #endif
>
> +/*
> + * A weight of 0, 1 or ULONG_MAX can cause arithmetics problems.
> + * (The default weight is 1024 - so there's no practical
> + * limitation from this.)
> + */
> #define MIN_SHARES 2
> +#define MAX_SHARES (ULONG_MAX - 1)
>
> static int init_task_group_load = INIT_TASK_GROUP_LOAD;
> #endif
> @@ -1748,6 +1754,8 @@ __update_group_shares_cpu(struct task_group *tg, struct sched_domain *sd,
>
> if (shares < MIN_SHARES)
> shares = MIN_SHARES;
> + else if (shares > MAX_SHARES)
> + shares = MAX_SHARES;
>
> __set_se_shares(tg->se[tcpu], shares);
> }
> @@ -8722,13 +8730,10 @@ int sched_group_set_shares(struct task_group *tg, unsigned long shares)
> if (!tg->se[0])
> return -EINVAL;
>
> - /*
> - * A weight of 0 or 1 can cause arithmetics problems.
> - * (The default weight is 1024 - so there's no practical
> - * limitation from this.)
> - */
> if (shares < MIN_SHARES)
> shares = MIN_SHARES;
> + else if (shares > MAX_SHARES)
> + shares = MAX_SHARES;
>
> mutex_lock(&shares_mutex);
> if (tg->shares == shares)
> @@ -8753,7 +8758,7 @@ int sched_group_set_shares(struct task_group *tg, unsigned long shares)
> * force a rebalance
> */
> cfs_rq_set_shares(tg->cfs_rq[i], 0);
> - set_se_shares(tg->se[i], shares/nr_cpu_ids);
> + set_se_shares(tg->se[i], shares);
> }
>
> /*

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