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: 29c1147	Previous: f83a235	Performance Ratio
startup_benchmark Build Time	75.96 s	77.08 s	0.99 ❗
startup_benchmark File Size	0.73 MB	0.73 MB	1.00 ❗
Startup Time - 1 core	0.74 s (±0.02 s)	0.73 s (±0.02 s)	1.02
Startup Time - 2 cores	0.73 s (±0.03 s)	0.75 s (±0.02 s)	0.97
Startup Time - 4 cores	0.74 s (±0.02 s)	0.76 s (±0.02 s)	0.98
multithreaded_benchmark Build Time	76.11 s	80.66 s	0.94 ❗
multithreaded_benchmark File Size	0.86 MB	0.86 MB	1
Multithreaded Pi Efficiency - 2 Threads	91.53 % (±7.12 %)	89.44 % (±5.91 %)	1.02
Multithreaded Pi Efficiency - 4 Threads	44.56 % (±2.76 %)	44.06 % (±2.65 %)	1.01
Multithreaded Pi Efficiency - 8 Threads	25.44 % (±1.46 %)	25.56 % (±1.89 %)	1.00
micro_benchmarks Build Time	74.51 s	87.20 s	0.85 ❗
micro_benchmarks File Size	0.87 MB	0.87 MB	1.00 ❗
Scheduling time - 1 thread	65.26 ticks (±2.72 ticks)	67.04 ticks (±1.91 ticks)	0.97
Scheduling time - 2 threads	38.10 ticks (±5.76 ticks)	39.33 ticks (±3.42 ticks)	0.97
Micro - Time for syscall (getpid)	3.99 ticks (±0.66 ticks)	3.43 ticks (±0.24 ticks)	1.17
Memcpy speed - (built_in) block size 4096	84167.33 MByte/s (±58178.09 MByte/s)	84527.91 MByte/s (±58322.62 MByte/s)	1.00
Memcpy speed - (built_in) block size 1048576	30768.34 MByte/s (±24749.35 MByte/s)	30845.87 MByte/s (±25037.16 MByte/s)	1.00
Memcpy speed - (built_in) block size 16777216	29810.46 MByte/s (±24496.86 MByte/s)	27492.52 MByte/s (±22708.20 MByte/s)	1.08
Memset speed - (built_in) block size 4096	84555.10 MByte/s (±58440.10 MByte/s)	84666.80 MByte/s (±58421.97 MByte/s)	1.00
Memset speed - (built_in) block size 1048576	31524.29 MByte/s (±25199.03 MByte/s)	31593.14 MByte/s (±25470.12 MByte/s)	1.00
Memset speed - (built_in) block size 16777216	30594.55 MByte/s (±24954.62 MByte/s)	28269.66 MByte/s (±23193.64 MByte/s)	1.08
Memcpy speed - (rust) block size 4096	76959.39 MByte/s (±53568.72 MByte/s)	74722.10 MByte/s (±52209.99 MByte/s)	1.03
Memcpy speed - (rust) block size 1048576	30638.61 MByte/s (±24712.72 MByte/s)	30824.11 MByte/s (±25054.28 MByte/s)	0.99
Memcpy speed - (rust) block size 16777216	29788.25 MByte/s (±24472.89 MByte/s)	27597.67 MByte/s (±22766.03 MByte/s)	1.08
Memset speed - (rust) block size 4096	77376.97 MByte/s (±53862.94 MByte/s)	74865.22 MByte/s (±52302.23 MByte/s)	1.03
Memset speed - (rust) block size 1048576	31388.27 MByte/s (±25155.84 MByte/s)	31565.59 MByte/s (±25484.76 MByte/s)	0.99
Memset speed - (rust) block size 16777216	30570.69 MByte/s (±24928.35 MByte/s)	28374.92 MByte/s (±23250.83 MByte/s)	1.08
alloc_benchmarks Build Time	73.66 s	81.01 s	0.91 ❗
alloc_benchmarks File Size	0.81 MB	0.81 MB	1.00 ❗
Allocations - Allocation success	91.32 %	91.32 %	1
Allocations - Deallocation success	100.00 %	100.00 %	1
Allocations - Pre-fail Allocations	61.46 %	61.46 %	1
Allocations - Average Allocation time	6249.06 Ticks (±176.82 Ticks)	6133.98 Ticks (±129.62 Ticks)	1.02
Allocations - Average Allocation time (no fail)	6820.31 Ticks (±175.75 Ticks)	6784.80 Ticks (±184.79 Ticks)	1.01
Allocations - Average Deallocation time	1228.31 Ticks (±310.46 Ticks)	1292.85 Ticks (±344.36 Ticks)	0.95
mutex_benchmark Build Time	74.23 s	81.90 s	0.91 ❗
mutex_benchmark File Size	0.87 MB	0.87 MB	1.00 ❗
Mutex Stress Test Average Time per Iteration - 1 Threads	12.02 ns (±0.47 ns)	12.08 ns (±0.27 ns)	1.00
Mutex Stress Test Average Time per Iteration - 2 Threads	38.56 ns (±2.27 ns)	17.94 ns (±3.62 ns)	2.15 ❗

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