MP2: RATE-MONOTONIC CPU SCHEDULING Based on slides by Gourav Khaneja, Raoul Rivas and Keun Yim University of Illinois at Urbana-Champaign Department of Computer Science CS423

G OAL  Understand real-time scheduling concepts  Design a real-time scheduler (i.e., RM) and its admission controller in OS  Implement the RM scheduler and admission controller as a Linux kernel module  Learn how to use the kernel-lv. API of the Linux scheduler, timer, and proc file system  Test the kernel-level real-time scheduler by implementing a user-level periodic task CS423 MP2 2 Extending Linux kernel to support RM scheduler

R ATE M ONOTONIC S CHEDULER  Periodic tasks to be completed before next period starts.  Medical systems such as pace maker, airline navigation system CS423 MP2 3

/P ROC  Uses the Proc file system as an interface instead of system calls  Process Registers: provide pid, period, processing time Yield: Finished Processing. Sleep until the next period De-register CS423 MP2 4

C ONTEXT S WITCH  Use Linux Scheduler for context switches through scheduling API and policies. CS423 MP2 5 Linux Scheduler User Space Proc FS Read Proc FS Write RMS Kernel Module Kernel Space Periodic App.

D ESIGN CS423 MP2 6 Process (or Task) Period Processing Time Deadline of Job 1 3. Job Arrival Job completion 4. Yield Kernel Space Proc FS Read Op. Proc FS Write Op. 2. List of RM Processes 1. Register Process Linked List for RM Tasks Dispatching Thread (High RT Priority) Timer RM Admission Controller Yield Func. Handler Linux Scheduler User- Level Periodic Process 4. Yield 5. Deregister 1. Register 2. Check wakeup() User Space … 5. Deregister

P ROCESS C ONTROL B LOCK  PCB is defined by the task_struct structure  PCBs are managed by a circular doubly linked list  task_struct *current points to PCB of the currently running process.  PCB contains the process state: Running or ready: TASK_RUNNING Waiting: TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE Other: TASK_TRACED, TASK_STOPPED CS423 MP2 7 task_struct Current

task_struct P ROCESS C ONTROL B LOCK  Additionally PCB contains: priority: the static priority rt_priority: the real-time priority counter: the number of clock ticks left in the scheduling budget of the process policy: the scheduling policy ○ SCHED_NORMAL: Normal ○ SCHED_FIFO or SCHED_RR mm: the memory descriptor CS423 MP2 8 priority rt_priority counter policy state need_ resched

M P 2_ TASK _ STRUCT  struct task_struct * task  Int period  Int processing_time  Int task_state  timer_list timer  … CS423 MP2 9

S CHEDULING P OINTS  schedule() is the entry point of the scheduler and invoked by: scheduler_tick() if a reschedule is marked. (task_struct→needs_resched) yield() is called by an application in user space schedule() in kernel or process context in kernel space  Can schedule() be called inside interrupt handler? CS423 MP2 10 schedule() Timer 1/HZ add_timer(1 jiffy) jiffies++ scheduler_tick() tick_periodic Kernel Contextyield() Process Context

T WO H ALVES / D EFERRING W ORK  Kernel timers are used to create timed events  They use jiffies to measure the system time  Timers are interrupts We can't sleep or hog CPU in them!  Solution: Divide the work into two parts Uses the timer handler to signal a thread. (TOP HALF) Let the kernel thread do the real job. (BOTTOM HALF) CS423 MP2 11 Timer Timer Handler: wake_up(thread); Thread: While(1) { Do work(); Schedule(); } Interrupt context Kernel context TOP HALF BOTTOM HALF

S CHEDULER API  schedule(): triggers rescheduling Runs after changing the state of any process  wake_up_process(struct task_struct *): This can be called in the interrupt context.  sched_setscheduler(): sets sched. parameters e.g., priority & scheduling policy  set_current_state(): changes the state used to put the running context to sleep.  set_task_state(): changes the state of another CS423 MP2 12

I MPLEMENTATION CS423 MP2 13 Select the “ready” process with shortest period. State = running Wake up process sleep State = ready Wake up scheduler Update timer State = sleep Wake up scheduler sleep Yield function handler Scheduler Timer interrupt

S CHEDULING H ACK  Use Linux Scheduling policies CS423 MP2 14 struct sched_param sparam; wake_up_process(task): sparam.sched_priority=MAX_USER_RT_PRIO-1; sched_setscheduler(task, SCHED_FIFO, &sparam);

LOCKS  Spinlocks inside timer interrupts  spin_lock_irqsave(spinlock_t *, unsigned ) Disable interrupts: can safely implement spinlocks on single core machines  spin_unlock_irqrestore(spinlock_t *, unsigned int)  Mutex for linked list CS423 MP2 15

M EMORY CACHE  Improves memory management by reducing fragmentation  For frequent allocation/de-allocation of given objects  Kmem_cache_create  Kmem_cache_alloc  Kmem_cache_free  Kmem_cache_destroy CS423 MP2 16

A DMISSION C ONTROL CS423 MP2 17

T EST APPLICATION void main (void) { REGISTER(PID, Period, ProcessTime); //Proc filesystem list=READ STATUS(); //Proc filesystem: Verify the process was admitted if (!process in the list) exit 1; //setup everything needed for real-time loop: t0=gettimeofday() for test.c YIELD(PID); //Proc filesystem //this is the real-time loop while(exist jobs) { do_job(); //wakeup_time=t0-gettimeofday() and factorial computation YIELD(PID); //Proc filesystem } UNREGISTER(PID); //Proc filesystem } CS423 MP2 18

T ESTS  Calculate the factorial of a fixed number  Estimate approx. processing time  Test with multiple applications  Verify Pre-emption CS423 MP2 19

C ONCLUSION  Linux uses Priority based Scheduling  Priorities are adjusted based on the state of the process and its scheduling policy  The PCB (task_struct) contains the information about the state of a process  Rescheduling in Linux follows the Two-Halves approach. Reschedule can occur in process context or kernel context only.  We can use the Scheduler API to build our own policy on top of the Linux scheduler.  Our RMS scheduler will have a scheduler thread and will be invoked after a Job Release and after a Job Completion CS423 MP2 20