Documentation/sched-coding.txt

   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
  18         inbalanced or periodically by the timer tick.  Prior to calling,
  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.