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Diffstat (limited to 'Documentation/sched-coding.txt')
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diff --git a/Documentation/sched-coding.txt b/Documentation/sched-coding.txt new file mode 100644 index 00000000000..2b75ef67c9f --- /dev/null +++ b/Documentation/sched-coding.txt @@ -0,0 +1,126 @@ + 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 + inbalanced 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. |