1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
//! Multilevel feedback queue scheduler for Tock
//!
//! Based on the MLFQ rules described in "Operating Systems: Three Easy Pieces"
//! by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau.
//!
//! This scheduler can be summarized by the following rules:
//!
//! - Rule 1: If Priority(A) > Priority(B), and both are ready, A runs (B
//!           doesn't).
//! - Rule 2: If Priority(A) = Priority(B), A & B run in round-robin fashion
//!           using the time slice (quantum length) of the given queue.
//! - Rule 3: When a job enters the system, it is placed at the highest priority
//!           (the topmost queue).
//! - Rule 4: Once a job uses up its time allotment at a given level (regardless
//!           of how many times it has given up the CPU), its priority is
//!           reduced (i.e., it moves down one queue).
//! - Rule 5: After some time period S, move all the jobs in the system to the
//!           topmost queue.

use crate::callback::AppId;
use crate::common::list::{List, ListLink, ListNode};
use crate::hil::time;
use crate::hil::time::Ticks;
use crate::platform::Chip;
use crate::process::ProcessType;
use crate::sched::{Kernel, Scheduler, SchedulingDecision, StoppedExecutingReason};
use core::cell::Cell;

#[derive(Default)]
struct MfProcState {
    /// Total CPU time used by this process while in current queue
    us_used_this_queue: Cell<u32>,
}

/// Nodes store per-process state
pub struct MLFQProcessNode<'a> {
    proc: &'static Option<&'static dyn ProcessType>,
    state: MfProcState,
    next: ListLink<'a, MLFQProcessNode<'a>>,
}

impl<'a> MLFQProcessNode<'a> {
    pub fn new(proc: &'static Option<&'static dyn ProcessType>) -> MLFQProcessNode<'a> {
        MLFQProcessNode {
            proc,
            state: MfProcState::default(),
            next: ListLink::empty(),
        }
    }
}

impl<'a> ListNode<'a, MLFQProcessNode<'a>> for MLFQProcessNode<'a> {
    fn next(&'a self) -> &'static ListLink<'a, MLFQProcessNode<'a>> {
        &self.next
    }
}

pub struct MLFQSched<'a, A: 'static + time::Alarm<'static>> {
    alarm: &'static A,
    pub processes: [List<'a, MLFQProcessNode<'a>>; 3], // Using Self::NUM_QUEUES causes rustc to crash..
    next_reset: Cell<A::Ticks>,
    last_reset_check: Cell<A::Ticks>,
    last_timeslice: Cell<u32>,
    last_queue_idx: Cell<usize>,
}

impl<'a, A: 'static + time::Alarm<'static>> MLFQSched<'a, A> {
    /// How often to restore all processes to max priority
    pub const PRIORITY_REFRESH_PERIOD_MS: u32 = 5000;
    pub const NUM_QUEUES: usize = 3;

    pub fn new(alarm: &'static A) -> Self {
        Self {
            alarm,
            processes: [List::new(), List::new(), List::new()],
            next_reset: Cell::new(A::Ticks::from(0)),
            last_reset_check: Cell::new(A::Ticks::from(0)),
            last_timeslice: Cell::new(0),
            last_queue_idx: Cell::new(0),
        }
    }

    fn get_timeslice_us(&self, queue_idx: usize) -> u32 {
        match queue_idx {
            0 => 10000,
            1 => 20000,
            2 => 50000,
            _ => panic!("invalid queue idx"),
        }
    }

    fn redeem_all_procs(&self) {
        let mut first = true;
        for queue in self.processes.iter() {
            if first {
                continue;
            }
            first = false;
            match queue.pop_head() {
                Some(proc) => self.processes[0].push_tail(proc),
                None => continue,
            }
        }
    }

    /// Returns the process at the head of the highest priority queue containing a process
    /// that is ready to execute (as determined by `has_tasks()`)
    /// This method moves that node to the head of its queue.
    fn get_next_ready_process_node(&self) -> (Option<&MLFQProcessNode<'a>>, usize) {
        for (idx, queue) in self.processes.iter().enumerate() {
            let next = queue
                .iter()
                .find(|node_ref| node_ref.proc.map_or(false, |proc| proc.ready()));
            if next.is_some() {
                // pop procs to back until we get to match
                loop {
                    let cur = queue.pop_head();
                    match cur {
                        Some(node) => {
                            if node as *const _ == next.unwrap() as *const _ {
                                queue.push_head(node);
                                // match! Put back on front
                                return (next, idx);
                            } else {
                                queue.push_tail(node);
                            }
                        }
                        None => {}
                    }
                }
            }
        }
        (None, 0)
    }
}

impl<'a, A: 'static + time::Alarm<'static>, C: Chip> Scheduler<C> for MLFQSched<'a, A> {
    fn next(&self, kernel: &Kernel) -> SchedulingDecision {
        if kernel.processes_blocked() {
            // No processes ready
            SchedulingDecision::TrySleep
        } else {
            let now = self.alarm.now();
            let next_reset = self.next_reset.get();
            let last_reset_check = self.last_reset_check.get();

            // storing last reset check is necessary to avoid missing a reset when the underlying
            // alarm wraps around
            if !now.within_range(last_reset_check, next_reset) {
                // Promote all processes to highest priority queue
                self.next_reset
                    .set(now.wrapping_add(A::ticks_from_ms(Self::PRIORITY_REFRESH_PERIOD_MS)));
                self.redeem_all_procs();
            }
            self.last_reset_check.set(now);
            let (node_ref_opt, queue_idx) = self.get_next_ready_process_node();
            let node_ref = node_ref_opt.unwrap(); // Panic if fail bc processes_blocked()!
            let timeslice =
                self.get_timeslice_us(queue_idx) - node_ref.state.us_used_this_queue.get();
            let next = node_ref.proc.unwrap().appid(); // Panic if fail bc processes_blocked()!
            self.last_queue_idx.set(queue_idx);
            self.last_timeslice.set(timeslice);

            SchedulingDecision::RunProcess((next, Some(timeslice)))
        }
    }

    fn result(&self, result: StoppedExecutingReason, execution_time_us: Option<u32>) {
        let execution_time_us = execution_time_us.unwrap(); // should never fail as we never run cooperatively
        let queue_idx = self.last_queue_idx.get();
        // Last executed node will always be at head of its queue
        let node_ref = self.processes[queue_idx].head().unwrap();
        node_ref
            .state
            .us_used_this_queue
            .set(self.last_timeslice.get() - execution_time_us);

        let punish = result == StoppedExecutingReason::TimesliceExpired;
        if punish {
            node_ref.state.us_used_this_queue.set(0);
            let next_queue = if queue_idx == Self::NUM_QUEUES - 1 {
                queue_idx
            } else {
                queue_idx + 1
            };
            self.processes[next_queue].push_tail(self.processes[queue_idx].pop_head().unwrap());
        } else {
            self.processes[queue_idx].push_tail(self.processes[queue_idx].pop_head().unwrap());
        }
    }

    unsafe fn continue_process(&self, _: AppId, _: &C) -> bool {
        // This MLFQ scheduler only preempts processes if there is a timeslice expiration
        true
    }
}