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root_domain是进程自动分组使用的root域。

/* * We add the notion of a root-domain which will be used to define per-domain * variables. Each exclusive cpuset essentially defines an island domain by * fully partitioning the member cpus from any other cpuset. Whenever a new * exclusive cpuset is created, we also create and attach a new root-domain * object. * */struct root_domain {	atomic_t refcount;	atomic_t rto_count;	struct rcu_head rcu;	cpumask_var_t span;	cpumask_var_t online;	/* Indicate more than one runnable task for any CPU */	bool overload;	/*	 * The bit corresponding to a CPU gets set here if such CPU has more	 * than one runnable -deadline task (as it is below for RT tasks).	 */	cpumask_var_t dlo_mask;	atomic_t dlo_count;	struct dl_bw dl_bw;	struct cpudl cpudl;	/*	 * The "RT overload" flag: it gets set if a CPU has more than	 * one runnable RT task.	 */	cpumask_var_t rto_mask;	struct cpupri cpupri;	unsigned long max_cpu_capacity;};

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