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1 Reference for various scheduler-related methods in the O(1) scheduler
2 Robert Love <rml@tech9.net>, MontaVista Software
3
4
5Note most of these methods are local to kernel/sched.c - this is by design.
6The scheduler is meant to be self-contained and abstracted away. This document
7is primarily for understanding the scheduler, not interfacing to it. Some of
8the discussed interfaces, however, are general process/scheduling methods.
9They are typically defined in include/linux/sched.h.
10
11
12Main Scheduling Methods
13-----------------------
14
15void load_balance(runqueue_t *this_rq, int idle)
16 Attempts to pull tasks from one cpu to another to balance cpu usage,
17 if needed. This method is called explicitly if the runqueues are
18 imbalanced or periodically by the timer tick. Prior to calling,
19 the current runqueue must be locked and interrupts disabled.
20
21void schedule()
22 The main scheduling function. Upon return, the highest priority
23 process will be active.
24
25
26Locking
27-------
28
29Each runqueue has its own lock, rq->lock. When multiple runqueues need
30to be locked, lock acquires must be ordered by ascending &runqueue value.
31
32A specific runqueue is locked via
33
34 task_rq_lock(task_t pid, unsigned long *flags)
35
36which disables preemption, disables interrupts, and locks the runqueue pid is
37running on. Likewise,
38
39 task_rq_unlock(task_t pid, unsigned long *flags)
40
41unlocks the runqueue pid is running on, restores interrupts to their previous
42state, and reenables preemption.
43
44The routines
45
46 double_rq_lock(runqueue_t *rq1, runqueue_t *rq2)
47
48and
49
50 double_rq_unlock(runqueue_t *rq1, runqueue_t *rq2)
51
52safely lock and unlock, respectively, the two specified runqueues. They do
53not, however, disable and restore interrupts. Users are required to do so
54manually before and after calls.
55
56
57Values
58------
59
60MAX_PRIO
61 The maximum priority of the system, stored in the task as task->prio.
62 Lower priorities are higher. Normal (non-RT) priorities range from
63 MAX_RT_PRIO to (MAX_PRIO - 1).
64MAX_RT_PRIO
65 The maximum real-time priority of the system. Valid RT priorities
66 range from 0 to (MAX_RT_PRIO - 1).
67MAX_USER_RT_PRIO
68 The maximum real-time priority that is exported to user-space. Should
69 always be equal to or less than MAX_RT_PRIO. Setting it less allows
70 kernel threads to have higher priorities than any user-space task.
71MIN_TIMESLICE
72MAX_TIMESLICE
73 Respectively, the minimum and maximum timeslices (quanta) of a process.
74
75Data
76----
77
78struct runqueue
79 The main per-CPU runqueue data structure.
80struct task_struct
81 The main per-process data structure.
82
83
84General Methods
85---------------
86
87cpu_rq(cpu)
88 Returns the runqueue of the specified cpu.
89this_rq()
90 Returns the runqueue of the current cpu.
91task_rq(pid)
92 Returns the runqueue which holds the specified pid.
93cpu_curr(cpu)
94 Returns the task currently running on the given cpu.
95rt_task(pid)
96 Returns true if pid is real-time, false if not.
97
98
99Process Control Methods
100-----------------------
101
102void set_user_nice(task_t *p, long nice)
103 Sets the "nice" value of task p to the given value.
104int setscheduler(pid_t pid, int policy, struct sched_param *param)
105 Sets the scheduling policy and parameters for the given pid.
106int set_cpus_allowed(task_t *p, unsigned long new_mask)
107 Sets a given task's CPU affinity and migrates it to a proper cpu.
108 Callers must have a valid reference to the task and assure the
109 task not exit prematurely. No locks can be held during the call.
110set_task_state(tsk, state_value)
111 Sets the given task's state to the given value.
112set_current_state(state_value)
113 Sets the current task's state to the given value.
114void set_tsk_need_resched(struct task_struct *tsk)
115 Sets need_resched in the given task.
116void clear_tsk_need_resched(struct task_struct *tsk)
117 Clears need_resched in the given task.
118void set_need_resched()
119 Sets need_resched in the current task.
120void clear_need_resched()
121 Clears need_resched in the current task.
122int need_resched()
123 Returns true if need_resched is set in the current task, false
124 otherwise.
125yield()
126 Place the current process at the end of the runqueue and call schedule.