linux-kernel - Re: [PATCH] sched: fair-group: fix a Div0 error of the fair group scheduler

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Date:	Mon, 28 Apr 2008 07:45:44 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	miaox@...fujitsu.com
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: fair-group: fix a Div0 error of the fair group
	scheduler

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.

how about:

diff --git a/kernel/sched.c b/kernel/sched.c
index 740fb40..b68127a 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -8025,7 +8025,7 @@ static void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
 
 	se->my_q = cfs_rq;
 	se->load.weight = tg->shares;
-	se->load.inv_weight = div64_64(1ULL<<32, se->load.weight);
+	se->load.inv_weight = 0;
 	se->parent = parent;
 }
 #endif
@@ -8692,7 +8692,7 @@ static void __set_se_shares(struct sched_entity *se, unsigned long shares)
 		dequeue_entity(cfs_rq, se, 0);
 
 	se->load.weight = shares;
-	se->load.inv_weight = div64_64((1ULL<<32), shares);
+	se->load.inv_weight = 0;
 
 	if (on_rq)
 		enqueue_entity(cfs_rq, se, 0);


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