x86_64: feature - preemptive multithreading

Cargo.toml

@@ -76,6 +76,7 @@ vga = []
 virtio = ["dep:virtio"]
 virtio-net = ["net", "virtio"]
 vsock = ["virtio", "pci"]
+preemptive-multithreading = []
 
 [lints.rust]
 rust_2018_idioms = "warn"

src/arch/x86_64/kernel/apic.rs

@@ -21,6 +21,8 @@ use x86_64::registers::control::Cr3;
 use x86_64::registers::model_specific::Msr;
 
 use super::interrupts::IDT;
+#[cfg(feature = "preemptive-multithreading")]
+use crate::arch::scheduler::MIN_RESCHEDULE_INTERVAL_US;
 use crate::arch::x86_64::kernel::CURRENT_STACK_ADDRESS;
 #[cfg(feature = "acpi")]
 use crate::arch::x86_64::kernel::acpi;
@@ -683,8 +685,26 @@ fn calibrate_timer() {
 	);
 }
 
-fn __set_oneshot_timer(wakeup_time: Option<u64>) {
-	if let Some(wt) = wakeup_time {
+fn __set_oneshot_timer_relative(ticks: u64) {
+	#[cfg(feature = "preemptive-multithreading")]
+	let ticks = cmp::min(ticks, MIN_RESCHEDULE_INTERVAL_US);
+
+	let ticks = cmp::min(
+		CALIBRATED_COUNTER_VALUE.get().unwrap() * ticks,
+		u64::from(u32::MAX),
+	);
+
+	__set_oneshot_timer_ticks(ticks);
+}
+
+fn __set_oneshot_timer_ticks(ticks: u64) {
+	// Enable the APIC Timer in One-Shot Mode and let it start by setting the initial counter value.
+	local_apic_write(IA32_X2APIC_LVT_TIMER, u64::from(TIMER_INTERRUPT_NUMBER));
+	local_apic_write(IA32_X2APIC_INIT_COUNT, ticks);
+}
+
+fn __set_oneshot_timer(wakeup_time_us: Option<u64>) {
+	if let Some(wt) = wakeup_time_us {
 		if processor::supports_tsc_deadline() {
 			// wt is the absolute wakeup time in microseconds based on processor::get_timer_ticks.
 			// We can simply multiply it by the processor frequency to get the absolute Time-Stamp Counter deadline
@@ -710,24 +730,28 @@ fn __set_oneshot_timer(wakeup_time: Option<u64>) {
 			} else {
 				1
 			};
-			let init_count = cmp::min(
-				CALIBRATED_COUNTER_VALUE.get().unwrap() * ticks,
-				u64::from(u32::MAX),
-			);
 
-			// Enable the APIC Timer in One-Shot Mode and let it start by setting the initial counter value.
-			local_apic_write(IA32_X2APIC_LVT_TIMER, u64::from(TIMER_INTERRUPT_NUMBER));
-			local_apic_write(IA32_X2APIC_INIT_COUNT, init_count);
+			__set_oneshot_timer_relative(ticks);
 		}
 	} else {
 		// Disable the APIC Timer.
+		#[cfg(not(feature = "preemptive-multithreading"))]
 		local_apic_write(IA32_X2APIC_LVT_TIMER, APIC_LVT_MASK);
 	}
 }
 
-pub fn set_oneshot_timer(wakeup_time: Option<u64>) {
+/// Sets a timer at precisely abs_wakeup_time_us after boot
+pub fn set_oneshot_timer(abs_wakeup_time_us: Option<u64>) {
+	without_interrupts(|| {
+		__set_oneshot_timer(abs_wakeup_time_us);
+	});
+}
+
+/// Sets a timer in precisely rel_wakeup_time_us microseconds$
+#[cfg(feature = "preemptive-multithreading")]
+pub fn set_oneshot_timer_relative(rel_wakeup_time_us: u64) {
 	without_interrupts(|| {
-		__set_oneshot_timer(wakeup_time);
+		__set_oneshot_timer_relative(rel_wakeup_time_us);
 	});
 }
 

src/arch/x86_64/kernel/scheduler.rs

@@ -14,6 +14,8 @@ use free_list::{PageLayout, PageRange};
 use memory_addresses::{PhysAddr, VirtAddr};
 
 use super::interrupts::{IDT, IST_SIZE};
+#[cfg(feature = "preemptive-multithreading")]
+use crate::arch::kernel::apic::set_oneshot_timer_relative;
 use crate::arch::x86_64::kernel::core_local::*;
 use crate::arch::x86_64::kernel::{apic, interrupts};
 use crate::arch::x86_64::mm::paging::{
@@ -26,6 +28,10 @@ use crate::mm::virtualmem::KERNEL_FREE_LIST;
 use crate::scheduler::PerCoreSchedulerExt;
 use crate::scheduler::task::{Task, TaskFrame};
 
+// Run scheduler at least every xxx micro-seconds
+#[cfg(feature = "preemptive-multithreading")]
+pub const MIN_RESCHEDULE_INTERVAL_US: u64 = 50;
+
 #[repr(C, packed)]
 struct State {
 	#[cfg(feature = "common-os")]
@@ -398,6 +404,8 @@ extern "x86-interrupt" fn timer_handler(_stack_frame: interrupts::ExceptionStack
 	increment_irq_counter(apic::TIMER_INTERRUPT_NUMBER);
 	core_scheduler().handle_waiting_tasks();
 	apic::eoi();
+	#[cfg(feature = "preemptive-multithreading")]
+	set_oneshot_timer_relative(MIN_RESCHEDULE_INTERVAL_US);
 	core_scheduler().reschedule();
 }
 
@@ -409,4 +417,7 @@ pub fn install_timer_handler() {
 			.set_stack_index(0);
 	}
 	interrupts::add_irq_name(apic::TIMER_INTERRUPT_NUMBER - 32, "Timer");
+
+	#[cfg(feature = "preemptive-multithreading")]
+	set_oneshot_timer_relative(1);
 }