Scheduling

CPU scheduling algorithms in MentOS.

Overview

The scheduler answers two separate questions, and it is worth keeping them apart from the very beginning:

Scheduling policy — which runnable task should run next? This is the algorithm (Round-Robin, CFS, EDF, ...). In MentOS it lives in kernel/src/process/scheduler_algorithm.c.
Context switch — how do we stop one task and resume another? Saving and restoring registers, switching the page directory, updating the TSS kernel stack. In MentOS this lives in kernel/src/process/scheduler.c and is the same code no matter which policy you pick.

Changing the policy changes nothing about the mechanism. Most of this page is about the policy.

Implementation status — read this first

MentOS lets you select a scheduling policy at configure time:

# kernel/CMakeLists.txt
set(SCHEDULER_TYPES SCHEDULER_RR SCHEDULER_PRIORITY SCHEDULER_CFS
                    SCHEDULER_EDF SCHEDULER_RM SCHEDULER_AEDF)
set(SCHEDULER_TYPE "SCHEDULER_RR" CACHE STRING "...")

Selectable is not the same as implemented. In the current tree:

Policy	CMake value	Status
Round-Robin	`SCHEDULER_RR` (default)	Complete. `__scheduler_rr()` is fully written.
Priority	`SCHEDULER_PRIORITY`	Exercise scaffold. The comparison and the `min` initialiser are literal `/.../` placeholders.
CFS	`SCHEDULER_CFS`	Exercise scaffold. The vruntime comparison is a `/.../` placeholder.
EDF	`SCHEDULER_EDF`	Exercise scaffold.
RM	`SCHEDULER_RM`	Exercise scaffold.
AEDF	`SCHEDULER_AEDF`	Exercise scaffold.

Each scaffold is written as:

static inline task_struct *__scheduler_cfs(runqueue_t *runqueue, bool_t skip_periodic)
{
#ifdef SCHEDULER_CFS
    time_t min = /*...*/;                 // <- the part you are asked to write
    /* ... */
    return next;
#else
    return __scheduler_rr(runqueue, skip_periodic);
#endif
}

Selecting a scaffolded policy intentionally causes the build to fail at the exercise placeholders until the missing implementation is completed. /*...*/ is a comment, so time_t min = /*...*/; reaches the compiler as time_t min = ; — a syntax error. For example:

$ cmake .. -DSCHEDULER_TYPE=SCHEDULER_CFS && make
kernel/src/process/scheduler_algorithm.c:151:33: error: expected expression before ';' token
  151 |     time_t min         = /*...*/;
      |                                 ^

That error is the assignment, not a broken checkout. Throughout MentOS, /* ... */ marks a deliberately incomplete section left for you to write; treat it as an exercise marker before suspecting a defect.

Two details worth reading correctly in the snippet above:

The #else return __scheduler_rr(...) arm is not a runtime fallback. It is compiled only when that policy is not selected, and in that configuration the function is never called — scheduler_pick_next_task() dispatches through an #if defined / #elif chain. Its job is to keep the unselected functions well-formed.
Selecting no policy at all is a different, also-loud failure: #error "You should enable a scheduling algorithm!".

This is deliberate: these are the exercises the course is built around. The algorithm descriptions below are therefore best read as specifications of what you are being asked to implement, with Round-Robin as the worked example.

Available Schedulers

1. Round-Robin (RR)

Status: implemented and default. __scheduler_rr().

Type: Time-sharing, preemptive
Best for: General-purpose, interactive systems

Algorithm:

Each process gets a fixed time slice (quantum)
Processes are arranged in a circular queue
When quantum expires, process is preempted and moved to back of queue
All processes get equal CPU time

Configuration:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_RR

Characteristics:

Fair: All processes get equal time
Simple: Easy to implement and understand
Low overhead: Minimal scheduling overhead
Poor for real-time: No priority or deadline support

Time Quantum: Driven by timer ticks. MentOS programs the PIT with TICKS_PER_SECOND = 1193 (kernel/inc/hardware/timer.h), so a tick is roughly 0.84 ms — considerably finer than the 100 Hz / 10 ms figure textbooks usually quote.

Implementation: __scheduler_rr() in kernel/src/process/scheduler_algorithm.c. It walks the runqueue starting from the node after the current task (wrapping once), and returns the first TASK_RUNNING entry — which is exactly what makes it round-robin rather than "always pick the head".

2. Priority-Based Scheduler

Status: exercise scaffold. __scheduler_priority() has /*...*/ placeholders; selecting SCHEDULER_PRIORITY makes the build fail there until you complete it. The doc comment above it in the source spells out the pitfall you are meant to find.

Type: Priority-based, preemptive
Best for: Systems with varied workload priorities

Algorithm:

Each process has a priority (lower value = higher priority)
Scheduler always picks highest priority ready process
Lower priority processes may starve if high-priority processes keep arriving
Same priority processes use round-robin

Configuration:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_PRIORITY

Characteristics:

Prioritization: Important processes run first
Responsive: High-priority tasks get CPU immediately
Starvation risk: Low-priority processes may never run
No fairness guarantees

Priority Levels:

0-99: Real-time priorities
100-139: Normal priorities

Setting Priority:

#include <sched.h>

struct sched_param param;
param.sched_priority = 50;
sched_setparam(pid, &param);

3. Completely Fair Scheduler (CFS)

Status: exercise scaffold. __scheduler_cfs() has /*...*/ placeholders; selecting SCHEDULER_CFS makes the build fail there until you complete it.

Type: Fair-share, time-based
Best for: Desktop/server systems, Linux-like behavior

Algorithm:

Tracks "virtual runtime" (vruntime) for each process
Always picks process with lowest vruntime
vruntime increases as process runs
Nice values affect vruntime increase rate
Uses a runqueue scan to select the minimum vruntime task

Configuration:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_CFS

Characteristics:

Fair: Each process gets proportional CPU time
Responsive: Good interactive performance
Scalable: Efficient for many processes
Complex: More sophisticated algorithm

Nice Values:

Range: -20 (highest priority) to +19 (lowest)
Default: 0
Setting nice value:
```
nice(10);  // Decrease priority by 10
```

4. Earliest Deadline First (EDF)

Status: exercise scaffold. Selecting SCHEDULER_EDF makes the build fail at the /*...*/ placeholders in __scheduler_edf() until you complete it.

Type: Real-time, dynamic priority
Best for: Real-time systems with explicit deadlines

Algorithm:

Each task has a deadline
Scheduler always picks task with earliest deadline
Optimal for single-CPU real-time scheduling
Preempts if a task with earlier deadline arrives

Configuration:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_EDF

Characteristics:

Optimal: Maximizes number of tasks meeting deadlines
Dynamic: Priorities change based on deadlines
Real-time: Suitable for hard real-time systems
Predictable: Behavior is deterministic

Setting Deadline:

struct sched_param param;
param.deadline = 1000;  // absolute deadline in ticks
sched_setparam(pid, &param);

5. Rate Monotonic (RM)

Status: exercise scaffold. Selecting SCHEDULER_RM makes the build fail at the /*...*/ placeholders in __scheduler_rm() until you complete it.

Type: Real-time, static priority
Best for: Periodic real-time tasks

Algorithm:

Each task has a fixed period
Priority is inversely proportional to period (shorter period = higher priority)
Static priorities never change
Preemptive

Configuration:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_RM

Characteristics:

Simple: Static priorities, easy to analyze
Optimal: Among static-priority algorithms
Predictable: Behavior is deterministic
Limited: Only works for periodic tasks

Setting Period:

struct sched_param param;
param.period = 100;  // period in ticks
sched_setparam(pid, &param);

6. Adaptive Earliest Deadline First (AEDF)

Status: exercise scaffold. Selecting SCHEDULER_AEDF makes the build fail at the /*...*/ placeholders in __scheduler_aedf() until you complete it.

Type: Real-time, adaptive
Best for: Mixed periodic/aperiodic real-time workloads

Algorithm:

Combines EDF with aperiodic task handling
Periodic tasks use EDF
Aperiodic tasks are scheduled in slack time
Adapts to varying workload

Configuration:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_AEDF

Characteristics:

Flexible: Handles both periodic and aperiodic tasks
Efficient: Maximizes CPU utilization
Complex: More sophisticated than pure EDF
Research-oriented: Experimental scheduler

Comparison Table

Scheduler	Type	Preemptive	Fair	Real-time	Complexity
RR	Time-sharing	Yes	Yes	No	Low
Priority	Priority	Yes	No	Partial	Low
CFS	Fair-share	Yes	Yes	No	Medium
EDF	Real-time	Yes	N/A	Yes	Medium
RM	Real-time	Yes	N/A	Yes	Low
AEDF	Real-time	Yes	Partial	Yes	High

Choosing a Scheduler

Use RR when

Building a general-purpose system
Want simplicity and fairness
Interactive processes (shell, editor)
Learning OS concepts

Use Priority when

Some tasks are more important than others
Need explicit control over task importance
Background tasks should yield to foreground

Use CFS when

Want Linux-like behavior
Need fairness with some priority control
Desktop or server workloads
Many processes with varying importance

Use EDF when

Have real-time requirements with deadlines
Tasks have explicit timing constraints
Need optimal real-time scheduling
Willing to specify deadlines explicitly

Use RM when

All tasks are periodic
Want static priority analysis
Need predictable, deterministic behavior
Simple real-time system

Use AEDF when

Have both periodic and aperiodic real-time tasks
Need flexible real-time scheduling
Researching advanced scheduling algorithms

Implementation Details

Scheduler Interface

All schedulers implement:

// Initialize scheduler
void scheduler_initialize(void);

// Pick next task to run
task_struct *scheduler_pick_next_task(runqueue_t *runqueue);

// Enqueue a runnable task
void scheduler_enqueue_task(task_struct *task);

// Dequeue a task
void scheduler_dequeue_task(task_struct *task);

// Run scheduler (called from timer/interrupt paths)
void scheduler_run(pt_regs_t *f);

Scheduler Location

Source: kernel/src/process/scheduler.c — runqueue management and context switching
Header: kernel/inc/process/scheduler.h — also defines MAX_PROCESSES (256)
Algorithm implementations: kernel/src/process/scheduler_algorithm.c — this is the file you edit when you implement a policy

The runqueue

// kernel/inc/process/scheduler.h
typedef struct runqueue {
    size_t num_active;      // number of queued processes
    size_t num_periodic;    // number of queued periodic processes
    list_head_t queue;      // the queue itself (intrusive list of task_struct)
    task_struct *curr;      // the currently running process
} runqueue_t;

There is a single flat list, not one queue per priority level. A policy is therefore a selection function over that list — which is why implementing a new policy means writing one function, not restructuring the data. Tasks are linked in through task_struct::run_list, and runqueue.curr is what __scheduler_rr() uses as its starting point.

num_periodic exists because MentOS supports periodic real-time tasks (see sched_setparam() and waitperiod()); the skip_periodic argument threaded through the policy functions lets the scheduler make a first pass that ignores them.

Context Switch

The scheduler is invoked from more places than the timer tick. scheduler_run() is called from:

Timer interrupt — kernel/src/hardware/timer.c
The int 0x80 handler — kernel/src/system/syscall.c, at the end of every system call, which is how a task that blocks in a syscall yields immediately
The generic exception path — kernel/src/descriptor_tables/exception.c
The page-fault handler — kernel/src/mem/page_fault.c
Signal delivery — kernel/src/system/signal.c

Saying "scheduling only happens on the timer tick" is therefore too strong.

Context switch process:

// Save current process state
scheduler_store_context(f, current_task);

// Pick next task
next_task = scheduler_pick_next_task(&runqueue);

// Restore next task state
scheduler_restore_context(next_task, f);

// Update current task pointer
current_task = next_task;

Scheduler Configuration

Time Quantum

MentOS bases scheduling decisions on timer ticks. TICKS_PER_SECOND is 1193 (kernel/inc/hardware/timer.h) — the PIT's base frequency is 1193182 Hz, and this is the divisor MentOS picks.

Priority Ranges

Modify priority ranges:

// kernel/inc/process/prio.h
#define MAX_RT_PRIO 100
#define MAX_PRIO    (MAX_RT_PRIO + NICE_WIDTH)

Performance Metrics

Measuring Scheduler Performance

// In kernel code
unsigned long start = timer_get_ticks();
scheduler_pick_next_task(&runqueue);
unsigned long end = timer_get_ticks();
pr_debug("Scheduler overhead: %lu ticks\n", end - start);

Typical Overheads

These are indicative textbook figures for the algorithms, not measurements of the MentOS implementations (five of which are not implemented yet). Use them to reason about asymptotics, not to predict MentOS's numbers.

Scheduler	Overhead (cycles)	O() complexity
RR	~100	O(1)
Priority	~200	O(1)
CFS	~500	O(log n)
EDF	~400	O(n) or O(log n)
RM	~100	O(1)
AEDF	~600	O(log n)

Debugging Schedulers

Enable Scheduler Logging

// kernel/src/process/scheduler.c
#define __DEBUG_LEVEL__ LOGLEVEL_DEBUG

// Logs scheduler decisions
pr_debug("Switching from PID %d to PID %d\n", old_pid, new_pid);

Monitor Process States

Use procfs entries such as /proc/<pid>/stat and scheduler feedback in /proc/feedback.

Testing Schedulers

Test programs in userspace/tests/:

t_periodic1.c - Single periodic task
t_periodic2.c - Multiple periodic tasks
t_periodic3.c - Mixed periodic/aperiodic
t_schedfb.c - Scheduler feedback test

Run tests:

make qemu
/bin/tests/t_periodic1

Custom Scheduler

MentOS keeps every policy in one file, so the cheapest way to add one is to follow the pattern the existing scaffolds use rather than creating a new source file:

Add a __scheduler_custom() function in kernel/src/process/scheduler_algorithm.c, guarded the same way as the others:

static inline task_struct *__scheduler_custom(runqueue_t *runqueue, bool_t skip_periodic)
{
#ifdef SCHEDULER_CUSTOM
    /* your selection logic; return a TASK_RUNNING entry from runqueue->queue */
#else
    return __scheduler_rr(runqueue, skip_periodic);
#endif
}

Add a branch for it in the #if defined(SCHEDULER_RR) / #elif ... chain inside scheduler_pick_next_task(), in the same file.
Append SCHEDULER_CUSTOM to SCHEDULER_TYPES in kernel/CMakeLists.txt. The existing list(FIND ...) validation and target_compile_definitions() then handle the rest — you do not need to add a new source file to the build.

Build:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_CUSTOM
make

Your function only has to answer "which task next?". Context saving/restoring, page-directory switching and TSS updates are already handled by scheduler_run() and are policy-independent.

Hands-On Exercises

Exercise 1: Observe Scheduling in Action

Goal: See which processes are scheduled and their CPU time.

Program - userspace/bin/sched_observer.c:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <time.h>

int main() {
    printf("=== Scheduling Observer ===\n");
    printf("Running 3 processes with same priority\n\n");
    
    // Fork 3 child processes
    for (int i = 0; i < 3; i++) {
        pid_t pid = fork();
        
        if (pid == 0) {
            // Child: busy loop with periodic print
            printf("[Child %d, PID %d] Started\n", i, getpid());
            
            for (int j = 0; j < 5; j++) {
                // Simulate work (tight loop)
                volatile int sum = 0;
                for (int k = 0; k < 100000; k++) {
                    sum += k;
                }
                
                printf("[Child %d, PID %d] Iteration %d\n", i, getpid(), j);
            }
            
            printf("[Child %d, PID %d] Done\n", i, getpid());
            exit(0);
        } else if (pid < 0) {
            printf("Fork failed!\n");
            return 1;
        }
    }
    
    // Parent: wait for all children
    printf("[Parent] Waiting for children...\n");
    for (int i = 0; i < 3; i++) {
        wait(NULL);
    }
    printf("[Parent] All children done\n");
    
    return 0;
}

Steps:

Build: make sched_observer
Run: /bin/sched_observer
Observe:
- Which child prints most often?
- How is CPU time distributed?
- Look for interleaved output (context switches)

Exercise 2: Measure Context Switches

Goal: Count how many times your process is rescheduled.

Program - userspace/bin/context_count.c:

#include <stdio.h>
#include <unistd.h>
#include <signal.h>

static volatile int switches = 0;
static volatile int iterations = 0;

void signal_handler(int sig) {
    switches++;
    // Signal indicates process was scheduled
}

int main() {
    printf("=== Context Switch Counter ===\n");
    printf("This process will count context switches\n");
    printf("Use ps to see process switching\n\n");
    
    // Install signal handler
    signal(SIGALRM, signal_handler);
    
    // Set alarm to fire every 100ms
    alarm(1);
    
    // Do work
    for (int i = 0; i < 1000000; i++) {
        iterations++;
        
        // Simulate variable-length computation
        for (volatile int j = 0; j < 100; j++);
    }
    
    printf("Completed %d iterations\n", iterations);
    printf("Estimated context switches: %d\n", switches);
    
    return 0;
}

Advanced: Use /bin/ps to see process status during execution

Exercise 3: Priority-Based Scheduling Test

Goal: Demonstrate how high-priority processes get more CPU time.

Program - userspace/bin/priority_test.c:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
    printf("=== Priority Scheduling Test ===\n");
    printf("If your kernel has priority scheduling,\n");
    printf("high-priority processes should run more.\n\n");
    
    // Create processes with different priorities
    for (int priority = 0; priority < 3; priority++) {
        pid_t pid = fork();
        
        if (pid == 0) {
            // Child process
            // Try to set priority (if nice() is implemented)
            if (priority > 0) {
                nice(priority * 5);  // Lower priority
            }
            
            printf("[PID %d, Priority %d] Starting work\n", 
                   getpid(), priority);
            
            // Do work
            volatile long sum = 0;
            for (int i = 0; i < 10000000; i++) {
                sum += i;
            }
            
            printf("[PID %d, Priority %d] Finished\n", 
                   getpid(), priority);
            exit(0);
        }
    }
    
    // Parent waits
    for (int i = 0; i < 3; i++) {
        wait(NULL);
    }
    
    printf("Test complete\n");
    return 0;
}

Observation:

If priority scheduling works, process 0 (highest priority) finishes first
Processes 1 and 2 may take much longer

Exercise 4: Switch Scheduler Algorithm

Goal: Test different scheduling algorithms.

Steps:

Build with Round-Robin:

cd build
cmake .. -DSCHEDULER_TYPE=SCHEDULER_RR
make
make qemu

Run your own test program and observe timing (there is no /bin/sched_observer in the tree — use the program from Exercise 1, or one of the t_periodic* tests)

Rebuild with Priority scheduler:

cmake .. -DSCHEDULER_TYPE=SCHEDULER_PRIORITY
make
make qemu

Run same test, compare behavior

Analysis:

RR: More balanced CPU distribution
Priority: High-priority processes get more CPU

What happens today: step 3 does not get as far as running. __scheduler_priority() is still an exercise scaffold, so configuring with SCHEDULER_PRIORITY makes the build stop at its /*...*/ placeholders:

kernel/src/process/scheduler_algorithm.c:107:33: error: expected expression before ';' token

Implementing __scheduler_priority() so that this exercise can be completed — and then produces the difference described above — is the assignment.

Exercise 5: Understand Preemption

Goal: See how preemption works in real-time.

Program - userspace/bin/preemption_demo.c:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <signal.h>

static volatile int preempted = 0;

void preempt_handler(int sig) {
    preempted++;
    printf("  [Preempted! Count: %d]\n", preempted);
}

int main() {
    printf("=== Preemption Demo ===\n");
    printf("This shows how kernel preempts long-running processes\n\n");
    
    // Install signal to catch preemption
    signal(SIGALRM, preempt_handler);
    alarm(1);  // Signal every second
    
    printf("[PID %d] Starting long computation...\n", getpid());
    printf("Watch how I'm interrupted!\n\n");
    
    // Very long loop with prints
    for (int i = 0; i < 50; i++) {
        printf("Iteration %d\n", i);
        
        // Simulate work
        for (volatile int j = 0; j < 50000000; j++);
        
        sleep(0);  // Yield to other processes
    }
    
    printf("\nDone! Was preempted %d times\n", preempted);
    return 0;
}

Challenge: Implement Fair Queuing

Advanced Goal: Understand how the scheduler maintains fairness

Research:

Read kernel/src/process/scheduler.c to understand current algorithm
Look at process queue management
Challenge: Modify scheduler to prioritize I/O-bound processes
Test with programs that mix computation and I/O

Uh oh!

Scheduling

Overview

Implementation status — read this first

Available Schedulers

1. Round-Robin (RR)

2. Priority-Based Scheduler

3. Completely Fair Scheduler (CFS)

4. Earliest Deadline First (EDF)

5. Rate Monotonic (RM)

6. Adaptive Earliest Deadline First (AEDF)

Comparison Table

Choosing a Scheduler

Use RR when

Use Priority when

Use CFS when

Use EDF when

Use RM when

Use AEDF when

Implementation Details

Scheduler Interface

Scheduler Location

The runqueue

Context Switch

Scheduler Configuration

Time Quantum

Priority Ranges

Performance Metrics

Measuring Scheduler Performance

Typical Overheads

Debugging Schedulers

Enable Scheduler Logging

Monitor Process States

Testing Schedulers

Custom Scheduler

Hands-On Exercises

Exercise 1: Observe Scheduling in Action

Exercise 2: Measure Context Switches

Exercise 3: Priority-Based Scheduling Test

Exercise 4: Switch Scheduler Algorithm

Exercise 5: Understand Preemption

Challenge: Implement Fair Queuing

Further Reading

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