[net-next-2.6.git] / Documentation / sched-coding.txt

     Reference for various scheduler-related methods in the O(1) scheduler
		Robert Love <rml@tech9.net>, MontaVista Software


Note most of these methods are local to kernel/sched.c - this is by design.
The scheduler is meant to be self-contained and abstracted away.  This document
is primarily for understanding the scheduler, not interfacing to it.  Some of
the discussed interfaces, however, are general process/scheduling methods.
They are typically defined in include/linux/sched.h.


Main Scheduling Methods
-----------------------

void load_balance(runqueue_t *this_rq, int idle)
	Attempts to pull tasks from one cpu to another to balance cpu usage,
	if needed.  This method is called explicitly if the runqueues are
	imbalanced or periodically by the timer tick.  Prior to calling,
	the current runqueue must be locked and interrupts disabled.

void schedule()
	The main scheduling function.  Upon return, the highest priority
	process will be active.


Locking
-------

Each runqueue has its own lock, rq->lock.  When multiple runqueues need
to be locked, lock acquires must be ordered by ascending &runqueue value.

A specific runqueue is locked via

	task_rq_lock(task_t pid, unsigned long *flags)

which disables preemption, disables interrupts, and locks the runqueue pid is
running on.  Likewise,

	task_rq_unlock(task_t pid, unsigned long *flags)

unlocks the runqueue pid is running on, restores interrupts to their previous
state, and reenables preemption.

The routines

	double_rq_lock(runqueue_t *rq1, runqueue_t *rq2)

and

	double_rq_unlock(runqueue_t *rq1, runqueue_t *rq2)

safely lock and unlock, respectively, the two specified runqueues.  They do
not, however, disable and restore interrupts.  Users are required to do so
manually before and after calls.


Values
------

MAX_PRIO
	The maximum priority of the system, stored in the task as task->prio.
	Lower priorities are higher.  Normal (non-RT) priorities range from
	MAX_RT_PRIO to (MAX_PRIO - 1).
MAX_RT_PRIO
	The maximum real-time priority of the system.  Valid RT priorities
	range from 0 to (MAX_RT_PRIO - 1).
MAX_USER_RT_PRIO
	The maximum real-time priority that is exported to user-space.  Should
	always be equal to or less than MAX_RT_PRIO.  Setting it less allows
	kernel threads to have higher priorities than any user-space task.
MIN_TIMESLICE
MAX_TIMESLICE
	Respectively, the minimum and maximum timeslices (quanta) of a process.

Data
----

struct runqueue
	The main per-CPU runqueue data structure.
struct task_struct
	The main per-process data structure.


General Methods
---------------

cpu_rq(cpu)
	Returns the runqueue of the specified cpu.
this_rq()
	Returns the runqueue of the current cpu.
task_rq(pid)
	Returns the runqueue which holds the specified pid.
cpu_curr(cpu)
	Returns the task currently running on the given cpu.
rt_task(pid)
	Returns true if pid is real-time, false if not.


Process Control Methods
-----------------------

void set_user_nice(task_t *p, long nice)
	Sets the "nice" value of task p to the given value.
int setscheduler(pid_t pid, int policy, struct sched_param *param)
	Sets the scheduling policy and parameters for the given pid.
int set_cpus_allowed(task_t *p, unsigned long new_mask)
	Sets a given task's CPU affinity and migrates it to a proper cpu.
	Callers must have a valid reference to the task and assure the
	task not exit prematurely.  No locks can be held during the call.
set_task_state(tsk, state_value)
	Sets the given task's state to the given value.
set_current_state(state_value)
	Sets the current task's state to the given value.
void set_tsk_need_resched(struct task_struct *tsk)
	Sets need_resched in the given task.
void clear_tsk_need_resched(struct task_struct *tsk)
	Clears need_resched in the given task.
void set_need_resched()
	Sets need_resched in the current task.
void clear_need_resched()
	Clears need_resched in the current task.
int need_resched()
	Returns true if need_resched is set in the current task, false
	otherwise.
yield()
	Place the current process at the end of the runqueue and call schedule.
Commit	Line	Data
1da177e4 LT	1	Reference for various scheduler-related methods in the O(1) scheduler
	2	Robert Love <rml@tech9.net>, MontaVista Software
	3
	4
	5	Note most of these methods are local to kernel/sched.c - this is by design.
	6	The scheduler is meant to be self-contained and abstracted away. This document
	7	is primarily for understanding the scheduler, not interfacing to it. Some of
	8	the discussed interfaces, however, are general process/scheduling methods.
	9	They are typically defined in include/linux/sched.h.
	10
	11
	12	Main Scheduling Methods
	13	-----------------------
	14
	15	void 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
2fe0ae78	18	imbalanced or periodically by the timer tick. Prior to calling,
1da177e4 LT	19	the current runqueue must be locked and interrupts disabled.
	20
	21	void schedule()
	22	The main scheduling function. Upon return, the highest priority
	23	process will be active.
	24
	25
	26	Locking
	27	-------
	28
	29	Each runqueue has its own lock, rq->lock. When multiple runqueues need
	30	to be locked, lock acquires must be ordered by ascending &runqueue value.
	31
	32	A specific runqueue is locked via
	33
	34	task_rq_lock(task_t pid, unsigned long *flags)
	35
	36	which disables preemption, disables interrupts, and locks the runqueue pid is
	37	running on. Likewise,
	38
	39	task_rq_unlock(task_t pid, unsigned long *flags)
	40
	41	unlocks the runqueue pid is running on, restores interrupts to their previous
	42	state, and reenables preemption.
	43
	44	The routines
	45
	46	double_rq_lock(runqueue_t rq1, runqueue_t rq2)
	47
	48	and
	49
	50	double_rq_unlock(runqueue_t rq1, runqueue_t rq2)
	51
	52	safely lock and unlock, respectively, the two specified runqueues. They do
	53	not, however, disable and restore interrupts. Users are required to do so
	54	manually before and after calls.
	55
	56
	57	Values
	58	------
	59
	60	MAX_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).
	64	MAX_RT_PRIO
	65	The maximum real-time priority of the system. Valid RT priorities
	66	range from 0 to (MAX_RT_PRIO - 1).
	67	MAX_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.
	71	MIN_TIMESLICE
	72	MAX_TIMESLICE
	73	Respectively, the minimum and maximum timeslices (quanta) of a process.
	74
	75	Data
	76	----
	77
	78	struct runqueue
	79	The main per-CPU runqueue data structure.
	80	struct task_struct
	81	The main per-process data structure.
	82
83
84	General Methods
85	---------------
86
87	cpu_rq(cpu)
88	Returns the runqueue of the specified cpu.
89	this_rq()
90	Returns the runqueue of the current cpu.
91	task_rq(pid)
92	Returns the runqueue which holds the specified pid.
93	cpu_curr(cpu)
94	Returns the task currently running on the given cpu.
95	rt_task(pid)
96	Returns true if pid is real-time, false if not.
97
98
99	Process Control Methods
100	-----------------------
101
102	void set_user_nice(task_t *p, long nice)
103	Sets the "nice" value of task p to the given value.
104	int setscheduler(pid_t pid, int policy, struct sched_param *param)
105	Sets the scheduling policy and parameters for the given pid.
106	int 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.
110	set_task_state(tsk, state_value)
111	Sets the given task's state to the given value.
112	set_current_state(state_value)
113	Sets the current task's state to the given value.
114	void set_tsk_need_resched(struct task_struct *tsk)
115	Sets need_resched in the given task.
116	void clear_tsk_need_resched(struct task_struct *tsk)
117	Clears need_resched in the given task.
118	void set_need_resched()
119	Sets need_resched in the current task.
120	void clear_need_resched()
121	Clears need_resched in the current task.
122	int need_resched()
123	Returns true if need_resched is set in the current task, false
124	otherwise.
125	yield()
126	Place the current process at the end of the runqueue and call schedule.