scheduler: add optional preemptive scheduling

[stlankes]

The scheduler is tickless (a TSC-deadline one-shot timer), so a CPU-bound task that never blocks or yields keeps its core indefinitely. Add an optional preemptive feature that round-robins between ready tasks via a time slice.

A new Source::Preemption timer slot is armed for now + 10ms whenever the scheduler switches to a task while others are ready, and whenever custom_wakeup makes a task ready (so a freshly woken task preempts a non-yielding runner soon). When the slice expires, the timer handler's existing reschedule() switches to the next ready task. With no contention nothing is armed, so the system stays tickless.

[github-actions]

Benchmark Results

Details

Benchmark	Current: e236bed	Previous: 2e23902	Performance Ratio
startup_benchmark Build Time	91.85 s	80.34 s	1.14 ❗
startup_benchmark File Size	0.78 MB	0.80 MB	0.98 ❗
Startup Time - 1 core	0.76 s (±0.03 s)	0.75 s (±0.02 s)	1.02
Startup Time - 2 cores	0.77 s (±0.03 s)	0.74 s (±0.02 s)	1.04 ❗
Startup Time - 4 cores	0.75 s (±0.02 s)	0.74 s (±0.02 s)	1.01
multithreaded_benchmark Build Time	93.68 s	82.11 s	1.14 ❗
multithreaded_benchmark File Size	0.88 MB	0.86 MB	1.03 ❗
Multithreaded Pi Efficiency - 2 Threads	88.55 % (±8.39 %)	85.89 % (±6.61 %)	1.03
Multithreaded Pi Efficiency - 4 Threads	44.21 % (±3.46 %)	43.43 % (±2.56 %)	1.02
Multithreaded Pi Efficiency - 8 Threads	25.75 % (±1.64 %)	25.76 % (±1.53 %)	1.00
micro_benchmarks Build Time	93.06 s	80.40 s	1.16 ❗
micro_benchmarks File Size	0.89 MB	0.86 MB	1.03 ❗
Scheduling time - 1 thread	70.49 ticks (±3.50 ticks)	62.65 ticks (±4.06 ticks)	1.13 ❗
Scheduling time - 2 threads	37.87 ticks (±3.12 ticks)	34.08 ticks (±4.10 ticks)	1.11
Micro - Time for syscall (getpid)	3.89 ticks (±0.52 ticks)	3.45 ticks (±0.58 ticks)	1.13
Memcpy speed - (built_in) block size 4096	74470.34 MByte/s (±51930.84 MByte/s)	82448.38 MByte/s (±56997.13 MByte/s)	0.90
Memcpy speed - (built_in) block size 1048576	29579.15 MByte/s (±24030.25 MByte/s)	30585.98 MByte/s (±24707.84 MByte/s)	0.97
Memcpy speed - (built_in) block size 16777216	25161.96 MByte/s (±20903.39 MByte/s)	26340.06 MByte/s (±21720.96 MByte/s)	0.96
Memset speed - (built_in) block size 4096	74859.10 MByte/s (±52170.99 MByte/s)	82292.76 MByte/s (±56891.50 MByte/s)	0.91
Memset speed - (built_in) block size 1048576	30282.45 MByte/s (±24452.37 MByte/s)	31323.85 MByte/s (±25145.86 MByte/s)	0.97
Memset speed - (built_in) block size 16777216	25886.80 MByte/s (±21358.68 MByte/s)	27104.68 MByte/s (±22209.94 MByte/s)	0.96
Memcpy speed - (rust) block size 4096	70847.21 MByte/s (±49425.80 MByte/s)	74097.96 MByte/s (±51811.44 MByte/s)	0.96
Memcpy speed - (rust) block size 1048576	29468.63 MByte/s (±24015.64 MByte/s)	30361.60 MByte/s (±24602.37 MByte/s)	0.97
Memcpy speed - (rust) block size 16777216	25943.98 MByte/s (±21571.40 MByte/s)	27625.34 MByte/s (±22806.88 MByte/s)	0.94
Memset speed - (rust) block size 4096	70954.43 MByte/s (±49499.07 MByte/s)	74373.47 MByte/s (±51976.48 MByte/s)	0.95
Memset speed - (rust) block size 1048576	30203.27 MByte/s (±24434.78 MByte/s)	31110.89 MByte/s (±25033.24 MByte/s)	0.97
Memset speed - (rust) block size 16777216	26680.99 MByte/s (±22021.65 MByte/s)	28386.93 MByte/s (±23265.03 MByte/s)	0.94
alloc_benchmarks Build Time	86.87 s	74.76 s	1.16 ❗
alloc_benchmarks File Size	0.86 MB	0.87 MB	0.98 ❗
Allocations - Allocation success	91.38 %	91.31 %	1.00 ❗
Allocations - Deallocation success	100.00 %	100.00 %	1
Allocations - Pre-fail Allocations	61.60 %	61.44 %	1.00 ❗
Allocations - Average Allocation time	10327.38 Ticks (±172.61 Ticks)	5860.58 Ticks (±98.43 Ticks)	1.76 ❗
Allocations - Average Allocation time (no fail)	10768.99 Ticks (±136.64 Ticks)	6554.81 Ticks (±92.86 Ticks)	1.64 ❗
Allocations - Average Deallocation time	2305.56 Ticks (±690.29 Ticks)	1805.01 Ticks (±250.35 Ticks)	1.28
mutex_benchmark Build Time	88.12 s	79.82 s	1.10 ❗
mutex_benchmark File Size	0.89 MB	0.86 MB	1.03 ❗
Mutex Stress Test Average Time per Iteration - 1 Threads	13.16 ns (±0.54 ns)	12.10 ns (±0.41 ns)	1.09 ❗
Mutex Stress Test Average Time per Iteration - 2 Threads	88.86 ns (±2.94 ns)	40.26 ns (±1.68 ns)	2.21 ❗

This comment was automatically generated by workflow using github-action-benchmark.

[Gelbpunkt]

Suggested change

	let deadline = processor::get_timer_ticks() + PREEMPTION_SLICE_US;
	timer_interrupts::create_timer_abs(timer_interrupts::Source::Preemption, deadline);
	timer_interrupts::create_timer(timer_interrupts::Source::Preemption, PREEMPTION_SLICE_US);

same for all the other calls. The function create_timer needs its #[cfg(feature = "net")] replaced with #[cfg(any(feature = "net", feature = "preemptive"))]

[mkroening]

Please address @Gelbpunkt's comment. Then, this is fine with me. 👍

[stlankes]

Why is feature net required?