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Subject: Re: set_task_rq bug?
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: vatsa@linux.vnet.ibm.com
Cc: Ingo Molnar <mingo@elte.hu>, aneesh.kumar@linux.vnet.ibm.com,
       dhaval@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
       Balbir Singh <balbir@in.ibm.com>
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Date: Wed, 02 Apr 2008 09:48:57 +0200
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On Wed, 2008-04-02 at 13:22 +0530, Srivatsa Vaddagiri wrote:
> For the case of CONFIG_RT_GROUP_SCHED and !CONFIG_FAIR_GROUP_SCHED, this
> piece of code seems to be wrong?
> 
> /* Change a task's cfs_rq and parent entity if it moves across
>  * CPUs/groups */
> static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
> {
> #ifdef CONFIG_FAIR_GROUP_SCHED
>         p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
>         p->se.parent = task_group(p)->se[cpu];
> #endif
> 
> #ifdef CONFIG_RT_GROUP_SCHED
>         p->rt.rt_rq  = task_group(p)->rt_rq[cpu];
>         p->rt.parent = task_group(p)->rt_se[cpu];
> #endif
> }
> 
> Basically for the above config case, a task's p->se.cfs_rq is never set,
> which seems to be incorrect, as lot of sched_fair.c (for ex:
> enqueue_task_fair) banks on the availability of this information. It should 
> atleast be set to &rq->cfs?

struct sched_entity {
        struct load_weight      load;           /* for load-balancing */
        struct rb_node          timeline_node;
#ifdef CONFIG_FAIR_GROUP_SCHED
        struct rb_node          deadline_node;
        u64                     vdeadline;
#endif
        struct list_head        group_node;
        unsigned int            on_rq;

        u64                     exec_start;
        u64                     sum_exec_runtime;
        u64                     vruntime;
        u64                     prev_sum_exec_runtime;

        u64                     last_wakeup;
        u64                     avg_overlap;

#ifdef CONFIG_SCHEDSTATS
	...
#endif

#ifdef CONFIG_FAIR_GROUP_SCHED
        struct sched_entity     *parent;
        /* rq on which this entity is (to be) queued: */
        struct cfs_rq           *cfs_rq;
        /* rq "owned" by this entity/group: */
        struct cfs_rq           *my_q;
#endif
};

with !CONFIG_FAIR_GROUP_SCHED se.cfs_rq isn't even there.