COP 5641 - Kernel and Device Driver Programming


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COP 5641 - Kernel and Device Driver Programming

A New Linux Scheduling Algorithm - Sporadic Server

Affected Code

task_struct fields:

int prio -
"effective" priority taken into account by the scheduler, possibly boosted by priority inheritance (caused by priority inversion):
see rt_mutex_setprio()

int static_prio -
nice priority ( -20...19 plus (MAX_RT_PRIO + 20) ) - i.e. user nice priority translated to kernel space:
see set_user_nice(), called by sys_nice() system call

int normal_prio -
normal priority of a task:
for realtime tasks this is the rt_priority value translated to kernel space, for regular tasks it is static_prio
see normal_prio() and __setscheduler()

unsigned int rt_priority -
priority as it appears from userspace:
this is assigned in exactly one place in __setscheduler(),
and is the priority value returned to the user from sched_getparam()

sched_param fields:

POSIX system interface:

View Modifications

Modifications to be made in order to implement a new scheduler (as of Linux 2.6.25)

Single Unix Specification v3
create main scheduler file sched_sporadic.c
implement new instance of struct sched_class called sporadic_sched_class
link new scheduling class into peer maintained linked list of process schedulers:

modify include/linux/sched.h to include support for sporadic server (as mandated by POSIX/The Single Unix Specification)

int sched_ss_granularity (tentative - non-standard; entity to be regulated - SCHED_PROCESS for SS or SCHED_THREAD for TSP)

map symbolic constant SCHED_SPORADIC to sporadic_sched_class in static void __setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio)
check for policy SCHED_SPORADIC in int set_setscheduler(struct task_struct *p, int policy, struct sched_param *param)
modify static void __setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio) to take generic parameter sched_param *param instead of implementation specific parameter int prio

Week 5

Week 6

MAX_PRIO, DEFAULT_PRIO, MAX_RT_PRIO
Priority range: 0-140 (including 0, excluding 140) (lower priority is better)
Realtime priority range (SCHED_FIFO, SCHED_RR): 0-100 (including 0, excluding 100)
Remaining priority range (SCHED_NORMAL, SCHED_BATCH): 100-140 (including 100, excluding 140)
Default priority: 120