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Understanding the linux kernel-ch7-Process Scheduling

2008-07-22 20:13 555 查看

7.3 Data Structures Used by the Scheduler
	· The runqueue Data Structure
		//kernel/sched.c
		/*
		* This is the main, per-CPU runqueue data structure.
		*
		* Locking rule: those places that want to lock multiple runqueues
		* (such as the load balancing or the thread migration code), lock
		* acquire operations must be ordered by ascending &runqueue.
		*/
		struct runqueue {
			spinlock_t lock;

			/*
			* nr_running and cpu_load should be in the same cacheline because
			* remote CPUs use both these fields when doing load calculation.
			*/
			unsigned long nr_running;
		#ifdef CONFIG_SMP
			unsigned long cpu_load;
		#endif
			unsigned long long nr_switches;

			/*
			* This is part of a global counter where only the total sum
			* over all CPUs matters. A task can increase this counter on
			* one CPU and if it got migrated afterwards it may decrease
			* it on another CPU. Always updated under the runqueue lock:
			*/
			unsigned long nr_uninterruptible;

			unsigned long expired_timestamp;
			unsigned long long timestamp_last_tick;
			task_t curr, idle;
			struct mm_struct *prev_mm;
			prio_array_t active, expired, arrays[2];
			int best_expired_prio;
			atomic_t nr_iowait;

		#ifdef CONFIG_SMP
			struct sched_domain *sd;

			/* For active balancing */
			int active_balance;
			int push_cpu;

			task_t *migration_thread;
			struct list_head migration_queue;
		#endif

		#ifdef CONFIG_SCHEDSTATS
			……
		#endif
		};
	· fields of task_struct Data Structure
		……
		static DEFINE_PER_CPU(struct runqueue, runqueues);

		#define for_each_domain(cpu, domain) /
			for (domain = cpu_rq(cpu)->sd; domain; domain = domain->parent)

		#define cpu_rq(cpu)	(&per_cpu(runqueues, (cpu)))
		#define this_rq()	(&__get_cpu_var(runqueues))
		#define task_rq(p)	cpu_rq(task_cpu(p))
		#define cpu_curr(cpu)	(cpu_rq(cpu)->curr)

		When a new process is created, sched_fork( ), invoked by copy_process( ),
		sets the time_slice field of both current (the parent) and p (the child)
		processes in the following way:
		p->time_slice = (current->time_slice + 1) >> 1;
		current->time_slice >>= 1;

7.4. Functions Used by the Scheduler
	The scheduler relies on several functions in order to do its work; the most important are:
	scheduler_tick( )
		Keeps the time_slice counter of current up-to-date
	try_to_wake_up( )
		Awakens a sleeping process
	recalc_task_prio( )
		Updates the dynamic priority of a process
	schedule( )
		Selects a new process to be executed
	load_balance()
		Keeps the runqueues of a multiprocessor system balanced

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