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
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
//! A scheduler is initialized with a fixed number of workers. Each worker is
//! driven by a thread. Each worker has a "core" which contains data such as the
//! run queue and other state. When `block_in_place` is called, the worker's
//! "core" is handed off to a new thread allowing the scheduler to continue to
//! make progress while the originating thread blocks.
//!
//! # Shutdown
//!
//! Shutting down the runtime involves the following steps:
//!
//!  1. The Shared::close method is called. This closes the inject queue and
//!     OwnedTasks instance and wakes up all worker threads.
//!
//!  2. Each worker thread observes the close signal next time it runs
//!     Core::maintenance by checking whether the inject queue is closed.
//!     The Core::is_shutdown flag is set to true.
//!
//!  3. The worker thread calls `pre_shutdown` in parallel. Here, the worker
//!     will keep removing tasks from OwnedTasks until it is empty. No new
//!     tasks can be pushed to the OwnedTasks during or after this step as it
//!     was closed in step 1.
//!
//!  5. The workers call Shared::shutdown to enter the single-threaded phase of
//!     shutdown. These calls will push their core to Shared::shutdown_cores,
//!     and the last thread to push its core will finish the shutdown procedure.
//!
//!  6. The local run queue of each core is emptied, then the inject queue is
//!     emptied.
//!
//! At this point, shutdown has completed. It is not possible for any of the
//! collections to contain any tasks at this point, as each collection was
//! closed first, then emptied afterwards.
//!
//! ## Spawns during shutdown
//!
//! When spawning tasks during shutdown, there are two cases:
//!
//!  * The spawner observes the OwnedTasks being open, and the inject queue is
//!    closed.
//!  * The spawner observes the OwnedTasks being closed and doesn't check the
//!    inject queue.
//!
//! The first case can only happen if the OwnedTasks::bind call happens before
//! or during step 1 of shutdown. In this case, the runtime will clean up the
//! task in step 3 of shutdown.
//!
//! In the latter case, the task was not spawned and the task is immediately
//! cancelled by the spawner.
//!
//! The correctness of shutdown requires both the inject queue and OwnedTasks
//! collection to have a closed bit. With a close bit on only the inject queue,
//! spawning could run in to a situation where a task is successfully bound long
//! after the runtime has shut down. With a close bit on only the OwnedTasks,
//! the first spawning situation could result in the notification being pushed
//! to the inject queue after step 6 of shutdown, which would leave a task in
//! the inject queue indefinitely. This would be a ref-count cycle and a memory
//! leak.

use crate::loom::sync::{Arc, Mutex};
use crate::runtime;
use crate::runtime::context;
use crate::runtime::scheduler::multi_thread::{
    idle, queue, Counters, Handle, Idle, Overflow, Parker, Stats, TraceStatus, Unparker,
};
use crate::runtime::scheduler::{inject, Defer, Lock};
use crate::runtime::task::OwnedTasks;
use crate::runtime::{
    blocking, coop, driver, scheduler, task, Config, SchedulerMetrics, WorkerMetrics,
};
use crate::util::atomic_cell::AtomicCell;
use crate::util::rand::{FastRand, RngSeedGenerator};

use std::cell::RefCell;
use std::task::Waker;
use std::time::Duration;

cfg_metrics! {
    mod metrics;
}

cfg_taskdump! {
    mod taskdump;
}

cfg_not_taskdump! {
    mod taskdump_mock;
}

/// A scheduler worker
pub(super) struct Worker {
    /// Reference to scheduler's handle
    handle: Arc<Handle>,

    /// Index holding this worker's remote state
    index: usize,

    /// Used to hand-off a worker's core to another thread.
    core: AtomicCell<Core>,
}

/// Core data
struct Core {
    /// Used to schedule bookkeeping tasks every so often.
    tick: u32,

    /// When a task is scheduled from a worker, it is stored in this slot. The
    /// worker will check this slot for a task **before** checking the run
    /// queue. This effectively results in the **last** scheduled task to be run
    /// next (LIFO). This is an optimization for improving locality which
    /// benefits message passing patterns and helps to reduce latency.
    lifo_slot: Option<Notified>,

    /// When `true`, locally scheduled tasks go to the LIFO slot. When `false`,
    /// they go to the back of the `run_queue`.
    lifo_enabled: bool,

    /// The worker-local run queue.
    run_queue: queue::Local<Arc<Handle>>,

    /// True if the worker is currently searching for more work. Searching
    /// involves attempting to steal from other workers.
    is_searching: bool,

    /// True if the scheduler is being shutdown
    is_shutdown: bool,

    /// True if the scheduler is being traced
    is_traced: bool,

    /// Parker
    ///
    /// Stored in an `Option` as the parker is added / removed to make the
    /// borrow checker happy.
    park: Option<Parker>,

    /// Per-worker runtime stats
    stats: Stats,

    /// How often to check the global queue
    global_queue_interval: u32,

    /// Fast random number generator.
    rand: FastRand,
}

/// State shared across all workers
pub(crate) struct Shared {
    /// Per-worker remote state. All other workers have access to this and is
    /// how they communicate between each other.
    remotes: Box<[Remote]>,

    /// Global task queue used for:
    ///  1. Submit work to the scheduler while **not** currently on a worker thread.
    ///  2. Submit work to the scheduler when a worker run queue is saturated
    pub(super) inject: inject::Shared<Arc<Handle>>,

    /// Coordinates idle workers
    idle: Idle,

    /// Collection of all active tasks spawned onto this executor.
    pub(crate) owned: OwnedTasks<Arc<Handle>>,

    /// Data synchronized by the scheduler mutex
    pub(super) synced: Mutex<Synced>,

    /// Cores that have observed the shutdown signal
    ///
    /// The core is **not** placed back in the worker to avoid it from being
    /// stolen by a thread that was spawned as part of `block_in_place`.
    #[allow(clippy::vec_box)] // we're moving an already-boxed value
    shutdown_cores: Mutex<Vec<Box<Core>>>,

    /// The number of cores that have observed the trace signal.
    pub(super) trace_status: TraceStatus,

    /// Scheduler configuration options
    config: Config,

    /// Collects metrics from the runtime.
    pub(super) scheduler_metrics: SchedulerMetrics,

    pub(super) worker_metrics: Box<[WorkerMetrics]>,

    /// Only held to trigger some code on drop. This is used to get internal
    /// runtime metrics that can be useful when doing performance
    /// investigations. This does nothing (empty struct, no drop impl) unless
    /// the `tokio_internal_mt_counters` cfg flag is set.
    _counters: Counters,
}

/// Data synchronized by the scheduler mutex
pub(crate) struct Synced {
    /// Synchronized state for `Idle`.
    pub(super) idle: idle::Synced,

    /// Synchronized state for `Inject`.
    pub(crate) inject: inject::Synced,
}

/// Used to communicate with a worker from other threads.
struct Remote {
    /// Steals tasks from this worker.
    pub(super) steal: queue::Steal<Arc<Handle>>,

    /// Unparks the associated worker thread
    unpark: Unparker,
}

/// Thread-local context
pub(crate) struct Context {
    /// Worker
    worker: Arc<Worker>,

    /// Core data
    core: RefCell<Option<Box<Core>>>,

    /// Tasks to wake after resource drivers are polled. This is mostly to
    /// handle yielded tasks.
    pub(crate) defer: Defer,
}

/// Starts the workers
pub(crate) struct Launch(Vec<Arc<Worker>>);

/// Running a task may consume the core. If the core is still available when
/// running the task completes, it is returned. Otherwise, the worker will need
/// to stop processing.
type RunResult = Result<Box<Core>, ()>;

/// A task handle
type Task = task::Task<Arc<Handle>>;

/// A notified task handle
type Notified = task::Notified<Arc<Handle>>;

/// Value picked out of thin-air. Running the LIFO slot a handful of times
/// seemms sufficient to benefit from locality. More than 3 times probably is
/// overweighing. The value can be tuned in the future with data that shows
/// improvements.
const MAX_LIFO_POLLS_PER_TICK: usize = 3;

pub(super) fn create(
    size: usize,
    park: Parker,
    driver_handle: driver::Handle,
    blocking_spawner: blocking::Spawner,
    seed_generator: RngSeedGenerator,
    config: Config,
) -> (Arc<Handle>, Launch) {
    let mut cores = Vec::with_capacity(size);
    let mut remotes = Vec::with_capacity(size);
    let mut worker_metrics = Vec::with_capacity(size);

    // Create the local queues
    for _ in 0..size {
        let (steal, run_queue) = queue::local();

        let park = park.clone();
        let unpark = park.unpark();
        let metrics = WorkerMetrics::from_config(&config);
        let stats = Stats::new(&metrics);

        cores.push(Box::new(Core {
            tick: 0,
            lifo_slot: None,
            lifo_enabled: !config.disable_lifo_slot,
            run_queue,
            is_searching: false,
            is_shutdown: false,
            is_traced: false,
            park: Some(park),
            global_queue_interval: stats.tuned_global_queue_interval(&config),
            stats,
            rand: FastRand::from_seed(config.seed_generator.next_seed()),
        }));

        remotes.push(Remote { steal, unpark });
        worker_metrics.push(metrics);
    }

    let (idle, idle_synced) = Idle::new(size);
    let (inject, inject_synced) = inject::Shared::new();

    let remotes_len = remotes.len();
    let handle = Arc::new(Handle {
        shared: Shared {
            remotes: remotes.into_boxed_slice(),
            inject,
            idle,
            owned: OwnedTasks::new(),
            synced: Mutex::new(Synced {
                idle: idle_synced,
                inject: inject_synced,
            }),
            shutdown_cores: Mutex::new(vec![]),
            trace_status: TraceStatus::new(remotes_len),
            config,
            scheduler_metrics: SchedulerMetrics::new(),
            worker_metrics: worker_metrics.into_boxed_slice(),
            _counters: Counters,
        },
        driver: driver_handle,
        blocking_spawner,
        seed_generator,
    });

    let mut launch = Launch(vec![]);

    for (index, core) in cores.drain(..).enumerate() {
        launch.0.push(Arc::new(Worker {
            handle: handle.clone(),
            index,
            core: AtomicCell::new(Some(core)),
        }));
    }

    (handle, launch)
}

#[track_caller]
pub(crate) fn block_in_place<F, R>(f: F) -> R
where
    F: FnOnce() -> R,
{
    // Try to steal the worker core back
    struct Reset {
        take_core: bool,
        budget: coop::Budget,
    }

    impl Drop for Reset {
        fn drop(&mut self) {
            with_current(|maybe_cx| {
                if let Some(cx) = maybe_cx {
                    if self.take_core {
                        let core = cx.worker.core.take();
                        let mut cx_core = cx.core.borrow_mut();
                        assert!(cx_core.is_none());
                        *cx_core = core;
                    }

                    // Reset the task budget as we are re-entering the
                    // runtime.
                    coop::set(self.budget);
                }
            });
        }
    }

    let mut had_entered = false;
    let mut take_core = false;

    let setup_result = with_current(|maybe_cx| {
        match (
            crate::runtime::context::current_enter_context(),
            maybe_cx.is_some(),
        ) {
            (context::EnterRuntime::Entered { .. }, true) => {
                // We are on a thread pool runtime thread, so we just need to
                // set up blocking.
                had_entered = true;
            }
            (
                context::EnterRuntime::Entered {
                    allow_block_in_place,
                },
                false,
            ) => {
                // We are on an executor, but _not_ on the thread pool.  That is
                // _only_ okay if we are in a thread pool runtime's block_on
                // method:
                if allow_block_in_place {
                    had_entered = true;
                    return Ok(());
                } else {
                    // This probably means we are on the current_thread runtime or in a
                    // LocalSet, where it is _not_ okay to block.
                    return Err(
                        "can call blocking only when running on the multi-threaded runtime",
                    );
                }
            }
            (context::EnterRuntime::NotEntered, true) => {
                // This is a nested call to block_in_place (we already exited).
                // All the necessary setup has already been done.
                return Ok(());
            }
            (context::EnterRuntime::NotEntered, false) => {
                // We are outside of the tokio runtime, so blocking is fine.
                // We can also skip all of the thread pool blocking setup steps.
                return Ok(());
            }
        }

        let cx = maybe_cx.expect("no .is_some() == false cases above should lead here");

        // Get the worker core. If none is set, then blocking is fine!
        let core = match cx.core.borrow_mut().take() {
            Some(core) => core,
            None => return Ok(()),
        };

        // We are taking the core from the context and sending it to another
        // thread.
        take_core = true;

        // The parker should be set here
        assert!(core.park.is_some());

        // In order to block, the core must be sent to another thread for
        // execution.
        //
        // First, move the core back into the worker's shared core slot.
        cx.worker.core.set(core);

        // Next, clone the worker handle and send it to a new thread for
        // processing.
        //
        // Once the blocking task is done executing, we will attempt to
        // steal the core back.
        let worker = cx.worker.clone();
        runtime::spawn_blocking(move || run(worker));
        Ok(())
    });

    if let Err(panic_message) = setup_result {
        panic!("{}", panic_message);
    }

    if had_entered {
        // Unset the current task's budget. Blocking sections are not
        // constrained by task budgets.
        let _reset = Reset {
            take_core,
            budget: coop::stop(),
        };

        crate::runtime::context::exit_runtime(f)
    } else {
        f()
    }
}

impl Launch {
    pub(crate) fn launch(mut self) {
        for worker in self.0.drain(..) {
            runtime::spawn_blocking(move || run(worker));
        }
    }
}

fn run(worker: Arc<Worker>) {
    struct AbortOnPanic;

    impl Drop for AbortOnPanic {
        fn drop(&mut self) {
            if std::thread::panicking() {
                eprintln!("worker thread panicking; aborting process");
                std::process::abort();
            }
        }
    }

    // Catching panics on worker threads in tests is quite tricky. Instead, when
    // debug assertions are enabled, we just abort the process.
    #[cfg(debug_assertions)]
    let _abort_on_panic = AbortOnPanic;

    // Acquire a core. If this fails, then another thread is running this
    // worker and there is nothing further to do.
    let core = match worker.core.take() {
        Some(core) => core,
        None => return,
    };

    let handle = scheduler::Handle::MultiThread(worker.handle.clone());

    crate::runtime::context::enter_runtime(&handle, true, |_| {
        // Set the worker context.
        let cx = scheduler::Context::MultiThread(Context {
            worker,
            core: RefCell::new(None),
            defer: Defer::new(),
        });

        context::set_scheduler(&cx, || {
            let cx = cx.expect_multi_thread();

            // This should always be an error. It only returns a `Result` to support
            // using `?` to short circuit.
            assert!(cx.run(core).is_err());

            // Check if there are any deferred tasks to notify. This can happen when
            // the worker core is lost due to `block_in_place()` being called from
            // within the task.
            cx.defer.wake();
        });
    });
}

impl Context {
    fn run(&self, mut core: Box<Core>) -> RunResult {
        // Reset `lifo_enabled` here in case the core was previously stolen from
        // a task that had the LIFO slot disabled.
        self.reset_lifo_enabled(&mut core);

        // Start as "processing" tasks as polling tasks from the local queue
        // will be one of the first things we do.
        core.stats.start_processing_scheduled_tasks();

        while !core.is_shutdown {
            self.assert_lifo_enabled_is_correct(&core);

            if core.is_traced {
                core = self.worker.handle.trace_core(core);
            }

            // Increment the tick
            core.tick();

            // Run maintenance, if needed
            core = self.maintenance(core);

            // First, check work available to the current worker.
            if let Some(task) = core.next_task(&self.worker) {
                core = self.run_task(task, core)?;
                continue;
            }

            // We consumed all work in the queues and will start searching for work.
            core.stats.end_processing_scheduled_tasks();

            // There is no more **local** work to process, try to steal work
            // from other workers.
            if let Some(task) = core.steal_work(&self.worker) {
                // Found work, switch back to processing
                core.stats.start_processing_scheduled_tasks();
                core = self.run_task(task, core)?;
            } else {
                // Wait for work
                core = if !self.defer.is_empty() {
                    self.park_timeout(core, Some(Duration::from_millis(0)))
                } else {
                    self.park(core)
                };
            }
        }

        core.pre_shutdown(&self.worker);

        // Signal shutdown
        self.worker.handle.shutdown_core(core);
        Err(())
    }

    fn run_task(&self, task: Notified, mut core: Box<Core>) -> RunResult {
        let task = self.worker.handle.shared.owned.assert_owner(task);

        // Make sure the worker is not in the **searching** state. This enables
        // another idle worker to try to steal work.
        core.transition_from_searching(&self.worker);

        self.assert_lifo_enabled_is_correct(&core);

        // Measure the poll start time. Note that we may end up polling other
        // tasks under this measurement. In this case, the tasks came from the
        // LIFO slot and are considered part of the current task for scheduling
        // purposes. These tasks inherent the "parent"'s limits.
        core.stats.start_poll();

        // Make the core available to the runtime context
        *self.core.borrow_mut() = Some(core);

        // Run the task
        coop::budget(|| {
            task.run();
            let mut lifo_polls = 0;

            // As long as there is budget remaining and a task exists in the
            // `lifo_slot`, then keep running.
            loop {
                // Check if we still have the core. If not, the core was stolen
                // by another worker.
                let mut core = match self.core.borrow_mut().take() {
                    Some(core) => core,
                    None => {
                        // In this case, we cannot call `reset_lifo_enabled()`
                        // because the core was stolen. The stealer will handle
                        // that at the top of `Context::run`
                        return Err(());
                    }
                };

                // Check for a task in the LIFO slot
                let task = match core.lifo_slot.take() {
                    Some(task) => task,
                    None => {
                        self.reset_lifo_enabled(&mut core);
                        core.stats.end_poll();
                        return Ok(core);
                    }
                };

                if !coop::has_budget_remaining() {
                    core.stats.end_poll();

                    // Not enough budget left to run the LIFO task, push it to
                    // the back of the queue and return.
                    core.run_queue.push_back_or_overflow(
                        task,
                        &*self.worker.handle,
                        &mut core.stats,
                    );
                    // If we hit this point, the LIFO slot should be enabled.
                    // There is no need to reset it.
                    debug_assert!(core.lifo_enabled);
                    return Ok(core);
                }

                // Track that we are about to run a task from the LIFO slot.
                lifo_polls += 1;
                super::counters::inc_lifo_schedules();

                // Disable the LIFO slot if we reach our limit
                //
                // In ping-ping style workloads where task A notifies task B,
                // which notifies task A again, continuously prioritizing the
                // LIFO slot can cause starvation as these two tasks will
                // repeatedly schedule the other. To mitigate this, we limit the
                // number of times the LIFO slot is prioritized.
                if lifo_polls >= MAX_LIFO_POLLS_PER_TICK {
                    core.lifo_enabled = false;
                    super::counters::inc_lifo_capped();
                }

                // Run the LIFO task, then loop
                *self.core.borrow_mut() = Some(core);
                let task = self.worker.handle.shared.owned.assert_owner(task);
                task.run();
            }
        })
    }

    fn reset_lifo_enabled(&self, core: &mut Core) {
        core.lifo_enabled = !self.worker.handle.shared.config.disable_lifo_slot;
    }

    fn assert_lifo_enabled_is_correct(&self, core: &Core) {
        debug_assert_eq!(
            core.lifo_enabled,
            !self.worker.handle.shared.config.disable_lifo_slot
        );
    }

    fn maintenance(&self, mut core: Box<Core>) -> Box<Core> {
        if core.tick % self.worker.handle.shared.config.event_interval == 0 {
            super::counters::inc_num_maintenance();

            core.stats.end_processing_scheduled_tasks();

            // Call `park` with a 0 timeout. This enables the I/O driver, timer, ...
            // to run without actually putting the thread to sleep.
            core = self.park_timeout(core, Some(Duration::from_millis(0)));

            // Run regularly scheduled maintenance
            core.maintenance(&self.worker);

            core.stats.start_processing_scheduled_tasks();
        }

        core
    }

    /// Parks the worker thread while waiting for tasks to execute.
    ///
    /// This function checks if indeed there's no more work left to be done before parking.
    /// Also important to notice that, before parking, the worker thread will try to take
    /// ownership of the Driver (IO/Time) and dispatch any events that might have fired.
    /// Whenever a worker thread executes the Driver loop, all waken tasks are scheduled
    /// in its own local queue until the queue saturates (ntasks > LOCAL_QUEUE_CAPACITY).
    /// When the local queue is saturated, the overflow tasks are added to the injection queue
    /// from where other workers can pick them up.
    /// Also, we rely on the workstealing algorithm to spread the tasks amongst workers
    /// after all the IOs get dispatched
    fn park(&self, mut core: Box<Core>) -> Box<Core> {
        if let Some(f) = &self.worker.handle.shared.config.before_park {
            f();
        }

        if core.transition_to_parked(&self.worker) {
            while !core.is_shutdown && !core.is_traced {
                core.stats.about_to_park();
                core = self.park_timeout(core, None);

                // Run regularly scheduled maintenance
                core.maintenance(&self.worker);

                if core.transition_from_parked(&self.worker) {
                    break;
                }
            }
        }

        if let Some(f) = &self.worker.handle.shared.config.after_unpark {
            f();
        }
        core
    }

    fn park_timeout(&self, mut core: Box<Core>, duration: Option<Duration>) -> Box<Core> {
        self.assert_lifo_enabled_is_correct(&core);

        // Take the parker out of core
        let mut park = core.park.take().expect("park missing");

        // Store `core` in context
        *self.core.borrow_mut() = Some(core);

        // Park thread
        if let Some(timeout) = duration {
            park.park_timeout(&self.worker.handle.driver, timeout);
        } else {
            park.park(&self.worker.handle.driver);
        }

        self.defer.wake();

        // Remove `core` from context
        core = self.core.borrow_mut().take().expect("core missing");

        // Place `park` back in `core`
        core.park = Some(park);

        if core.should_notify_others() {
            self.worker.handle.notify_parked_local();
        }

        core
    }

    pub(crate) fn defer(&self, waker: &Waker) {
        self.defer.defer(waker);
    }
}

impl Core {
    /// Increment the tick
    fn tick(&mut self) {
        self.tick = self.tick.wrapping_add(1);
    }

    /// Return the next notified task available to this worker.
    fn next_task(&mut self, worker: &Worker) -> Option<Notified> {
        if self.tick % self.global_queue_interval == 0 {
            // Update the global queue interval, if needed
            self.tune_global_queue_interval(worker);

            worker
                .handle
                .next_remote_task()
                .or_else(|| self.next_local_task())
        } else {
            let maybe_task = self.next_local_task();

            if maybe_task.is_some() {
                return maybe_task;
            }

            if worker.inject().is_empty() {
                return None;
            }

            // Other threads can only **remove** tasks from the current worker's
            // `run_queue`. So, we can be confident that by the time we call
            // `run_queue.push_back` below, there will be *at least* `cap`
            // available slots in the queue.
            let cap = usize::min(
                self.run_queue.remaining_slots(),
                self.run_queue.max_capacity() / 2,
            );

            // The worker is currently idle, pull a batch of work from the
            // injection queue. We don't want to pull *all* the work so other
            // workers can also get some.
            let n = usize::min(
                worker.inject().len() / worker.handle.shared.remotes.len() + 1,
                cap,
            );

            // Take at least one task since the first task is returned directly
            // and nto pushed onto the local queue.
            let n = usize::max(1, n);

            let mut synced = worker.handle.shared.synced.lock();
            // safety: passing in the correct `inject::Synced`.
            let mut tasks = unsafe { worker.inject().pop_n(&mut synced.inject, n) };

            // Pop the first task to return immedietly
            let ret = tasks.next();

            // Push the rest of the on the run queue
            self.run_queue.push_back(tasks);

            ret
        }
    }

    fn next_local_task(&mut self) -> Option<Notified> {
        self.lifo_slot.take().or_else(|| self.run_queue.pop())
    }

    /// Function responsible for stealing tasks from another worker
    ///
    /// Note: Only if less than half the workers are searching for tasks to steal
    /// a new worker will actually try to steal. The idea is to make sure not all
    /// workers will be trying to steal at the same time.
    fn steal_work(&mut self, worker: &Worker) -> Option<Notified> {
        if !self.transition_to_searching(worker) {
            return None;
        }

        let num = worker.handle.shared.remotes.len();
        // Start from a random worker
        let start = self.rand.fastrand_n(num as u32) as usize;

        for i in 0..num {
            let i = (start + i) % num;

            // Don't steal from ourself! We know we don't have work.
            if i == worker.index {
                continue;
            }

            let target = &worker.handle.shared.remotes[i];
            if let Some(task) = target
                .steal
                .steal_into(&mut self.run_queue, &mut self.stats)
            {
                return Some(task);
            }
        }

        // Fallback on checking the global queue
        worker.handle.next_remote_task()
    }

    fn transition_to_searching(&mut self, worker: &Worker) -> bool {
        if !self.is_searching {
            self.is_searching = worker.handle.shared.idle.transition_worker_to_searching();
        }

        self.is_searching
    }

    fn transition_from_searching(&mut self, worker: &Worker) {
        if !self.is_searching {
            return;
        }

        self.is_searching = false;
        worker.handle.transition_worker_from_searching();
    }

    fn has_tasks(&self) -> bool {
        self.lifo_slot.is_some() || self.run_queue.has_tasks()
    }

    fn should_notify_others(&self) -> bool {
        // If there are tasks available to steal, but this worker is not
        // looking for tasks to steal, notify another worker.
        if self.is_searching {
            return false;
        }
        self.lifo_slot.is_some() as usize + self.run_queue.len() > 1
    }

    /// Prepares the worker state for parking.
    ///
    /// Returns true if the transition happened, false if there is work to do first.
    fn transition_to_parked(&mut self, worker: &Worker) -> bool {
        // Workers should not park if they have work to do
        if self.has_tasks() || self.is_traced {
            return false;
        }

        // When the final worker transitions **out** of searching to parked, it
        // must check all the queues one last time in case work materialized
        // between the last work scan and transitioning out of searching.
        let is_last_searcher = worker.handle.shared.idle.transition_worker_to_parked(
            &worker.handle.shared,
            worker.index,
            self.is_searching,
        );

        // The worker is no longer searching. Setting this is the local cache
        // only.
        self.is_searching = false;

        if is_last_searcher {
            worker.handle.notify_if_work_pending();
        }

        true
    }

    /// Returns `true` if the transition happened.
    fn transition_from_parked(&mut self, worker: &Worker) -> bool {
        // If a task is in the lifo slot/run queue, then we must unpark regardless of
        // being notified
        if self.has_tasks() {
            // When a worker wakes, it should only transition to the "searching"
            // state when the wake originates from another worker *or* a new task
            // is pushed. We do *not* want the worker to transition to "searching"
            // when it wakes when the I/O driver receives new events.
            self.is_searching = !worker
                .handle
                .shared
                .idle
                .unpark_worker_by_id(&worker.handle.shared, worker.index);
            return true;
        }

        if worker
            .handle
            .shared
            .idle
            .is_parked(&worker.handle.shared, worker.index)
        {
            return false;
        }

        // When unparked, the worker is in the searching state.
        self.is_searching = true;
        true
    }

    /// Runs maintenance work such as checking the pool's state.
    fn maintenance(&mut self, worker: &Worker) {
        self.stats
            .submit(&worker.handle.shared.worker_metrics[worker.index]);

        if !self.is_shutdown {
            // Check if the scheduler has been shutdown
            let synced = worker.handle.shared.synced.lock();
            self.is_shutdown = worker.inject().is_closed(&synced.inject);
        }

        if !self.is_traced {
            // Check if the worker should be tracing.
            self.is_traced = worker.handle.shared.trace_status.trace_requested();
        }
    }

    /// Signals all tasks to shut down, and waits for them to complete. Must run
    /// before we enter the single-threaded phase of shutdown processing.
    fn pre_shutdown(&mut self, worker: &Worker) {
        // Signal to all tasks to shut down.
        worker.handle.shared.owned.close_and_shutdown_all();

        self.stats
            .submit(&worker.handle.shared.worker_metrics[worker.index]);
    }

    /// Shuts down the core.
    fn shutdown(&mut self, handle: &Handle) {
        // Take the core
        let mut park = self.park.take().expect("park missing");

        // Drain the queue
        while self.next_local_task().is_some() {}

        park.shutdown(&handle.driver);
    }

    fn tune_global_queue_interval(&mut self, worker: &Worker) {
        let next = self
            .stats
            .tuned_global_queue_interval(&worker.handle.shared.config);

        debug_assert!(next > 1);

        // Smooth out jitter
        if abs_diff(self.global_queue_interval, next) > 2 {
            self.global_queue_interval = next;
        }
    }
}

impl Worker {
    /// Returns a reference to the scheduler's injection queue.
    fn inject(&self) -> &inject::Shared<Arc<Handle>> {
        &self.handle.shared.inject
    }
}

// TODO: Move `Handle` impls into handle.rs
impl task::Schedule for Arc<Handle> {
    fn release(&self, task: &Task) -> Option<Task> {
        self.shared.owned.remove(task)
    }

    fn schedule(&self, task: Notified) {
        self.schedule_task(task, false);
    }

    fn yield_now(&self, task: Notified) {
        self.schedule_task(task, true);
    }
}

impl Handle {
    pub(super) fn schedule_task(&self, task: Notified, is_yield: bool) {
        with_current(|maybe_cx| {
            if let Some(cx) = maybe_cx {
                // Make sure the task is part of the **current** scheduler.
                if self.ptr_eq(&cx.worker.handle) {
                    // And the current thread still holds a core
                    if let Some(core) = cx.core.borrow_mut().as_mut() {
                        self.schedule_local(core, task, is_yield);
                        return;
                    }
                }
            }

            // Otherwise, use the inject queue.
            self.push_remote_task(task);
            self.notify_parked_remote();
        })
    }

    pub(super) fn schedule_option_task_without_yield(&self, task: Option<Notified>) {
        if let Some(task) = task {
            self.schedule_task(task, false);
        }
    }

    fn schedule_local(&self, core: &mut Core, task: Notified, is_yield: bool) {
        core.stats.inc_local_schedule_count();

        // Spawning from the worker thread. If scheduling a "yield" then the
        // task must always be pushed to the back of the queue, enabling other
        // tasks to be executed. If **not** a yield, then there is more
        // flexibility and the task may go to the front of the queue.
        let should_notify = if is_yield || !core.lifo_enabled {
            core.run_queue
                .push_back_or_overflow(task, self, &mut core.stats);
            true
        } else {
            // Push to the LIFO slot
            let prev = core.lifo_slot.take();
            let ret = prev.is_some();

            if let Some(prev) = prev {
                core.run_queue
                    .push_back_or_overflow(prev, self, &mut core.stats);
            }

            core.lifo_slot = Some(task);

            ret
        };

        // Only notify if not currently parked. If `park` is `None`, then the
        // scheduling is from a resource driver. As notifications often come in
        // batches, the notification is delayed until the park is complete.
        if should_notify && core.park.is_some() {
            self.notify_parked_local();
        }
    }

    fn next_remote_task(&self) -> Option<Notified> {
        if self.shared.inject.is_empty() {
            return None;
        }

        let mut synced = self.shared.synced.lock();
        // safety: passing in correct `idle::Synced`
        unsafe { self.shared.inject.pop(&mut synced.inject) }
    }

    fn push_remote_task(&self, task: Notified) {
        self.shared.scheduler_metrics.inc_remote_schedule_count();

        let mut synced = self.shared.synced.lock();
        // safety: passing in correct `idle::Synced`
        unsafe {
            self.shared.inject.push(&mut synced.inject, task);
        }
    }

    pub(super) fn close(&self) {
        if self
            .shared
            .inject
            .close(&mut self.shared.synced.lock().inject)
        {
            self.notify_all();
        }
    }

    fn notify_parked_local(&self) {
        super::counters::inc_num_inc_notify_local();

        if let Some(index) = self.shared.idle.worker_to_notify(&self.shared) {
            super::counters::inc_num_unparks_local();
            self.shared.remotes[index].unpark.unpark(&self.driver);
        }
    }

    fn notify_parked_remote(&self) {
        if let Some(index) = self.shared.idle.worker_to_notify(&self.shared) {
            self.shared.remotes[index].unpark.unpark(&self.driver);
        }
    }

    pub(super) fn notify_all(&self) {
        for remote in &self.shared.remotes[..] {
            remote.unpark.unpark(&self.driver);
        }
    }

    fn notify_if_work_pending(&self) {
        for remote in &self.shared.remotes[..] {
            if !remote.steal.is_empty() {
                self.notify_parked_local();
                return;
            }
        }

        if !self.shared.inject.is_empty() {
            self.notify_parked_local();
        }
    }

    fn transition_worker_from_searching(&self) {
        if self.shared.idle.transition_worker_from_searching() {
            // We are the final searching worker. Because work was found, we
            // need to notify another worker.
            self.notify_parked_local();
        }
    }

    /// Signals that a worker has observed the shutdown signal and has replaced
    /// its core back into its handle.
    ///
    /// If all workers have reached this point, the final cleanup is performed.
    fn shutdown_core(&self, core: Box<Core>) {
        let mut cores = self.shared.shutdown_cores.lock();
        cores.push(core);

        if cores.len() != self.shared.remotes.len() {
            return;
        }

        debug_assert!(self.shared.owned.is_empty());

        for mut core in cores.drain(..) {
            core.shutdown(self);
        }

        // Drain the injection queue
        //
        // We already shut down every task, so we can simply drop the tasks.
        while let Some(task) = self.next_remote_task() {
            drop(task);
        }
    }

    fn ptr_eq(&self, other: &Handle) -> bool {
        std::ptr::eq(self, other)
    }
}

impl Overflow<Arc<Handle>> for Handle {
    fn push(&self, task: task::Notified<Arc<Handle>>) {
        self.push_remote_task(task);
    }

    fn push_batch<I>(&self, iter: I)
    where
        I: Iterator<Item = task::Notified<Arc<Handle>>>,
    {
        unsafe {
            self.shared.inject.push_batch(self, iter);
        }
    }
}

pub(crate) struct InjectGuard<'a> {
    lock: crate::loom::sync::MutexGuard<'a, Synced>,
}

impl<'a> AsMut<inject::Synced> for InjectGuard<'a> {
    fn as_mut(&mut self) -> &mut inject::Synced {
        &mut self.lock.inject
    }
}

impl<'a> Lock<inject::Synced> for &'a Handle {
    type Handle = InjectGuard<'a>;

    fn lock(self) -> Self::Handle {
        InjectGuard {
            lock: self.shared.synced.lock(),
        }
    }
}

#[track_caller]
fn with_current<R>(f: impl FnOnce(Option<&Context>) -> R) -> R {
    use scheduler::Context::MultiThread;

    context::with_scheduler(|ctx| match ctx {
        Some(MultiThread(ctx)) => f(Some(ctx)),
        _ => f(None),
    })
}

// `u32::abs_diff` is not available on Tokio's MSRV.
fn abs_diff(a: u32, b: u32) -> u32 {
    if a > b {
        a - b
    } else {
        b - a
    }
}