scx_rustland_core: Allow to define a default time slice

When initializing the BpfScheduler, allow specifying a default time
slice for tasks automatically dispatched by the scx_rustland_core
backend (for example, when the built-in idle CPU selection policy is
enabled and a task is automatically dispatched on an idle CPU).

This helps improve responsiveness for schedulers with latency-sensitive
requirements, ensuring that tasks never receive a time slice larger than
the value defined by the user-space scheduler.

Also bump up scx_rustland_core major version, as this change breaks
backward compatibility due to the new slice_ns argument added to
BpfScheduler.

Signed-off-by: Andrea Righi <[email protected]>

Author: arighi

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "scx_rustland_core"
-version = "2.3.7"
+version = "2.4.7"
 edition = "2021"
 authors = ["Andrea Righi <[email protected]>"]
 license = "GPL-2.0-only"

rust/scx_rustland_core/Cargo.toml

@@ -213,6 +213,7 @@ impl<'cb> BpfScheduler<'cb> {
         partial: bool,
         debug: bool,
         builtin_idle: bool,
+        slice_ns: u64,
         name: &str,
     ) -> Result<Self> {
         let shutdown = Arc::new(AtomicBool::new(false));
@@ -263,6 +264,7 @@ impl<'cb> BpfScheduler<'cb> {
         skel.maps.rodata_data.as_mut().unwrap().usersched_pid = std::process::id();
         skel.maps.rodata_data.as_mut().unwrap().khugepaged_pid = Self::khugepaged_pid();
         skel.maps.rodata_data.as_mut().unwrap().builtin_idle = builtin_idle;
+        skel.maps.rodata_data.as_mut().unwrap().slice_ns = slice_ns;
         skel.maps.rodata_data.as_mut().unwrap().debug = debug;
         let _ = Self::set_scx_ops_name(&mut skel.struct_ops.rustland_mut().name, name);
 

rust/scx_rustland_core/assets/bpf.rs

@@ -68,6 +68,11 @@ const volatile u32 khugepaged_pid; /* khugepaged PID */
 u64 usersched_last_run_at; /* Timestamp of the last user-space scheduler execution */
 static u64 nr_cpu_ids; /* Maximum possible CPU number */
 
+/*
+ * Default task time slice.
+ */
+const volatile u64 slice_ns;
+
 /*
  * Number of tasks that are queued for scheduling.
  *
@@ -587,7 +592,7 @@ s32 BPF_STRUCT_OPS(rustland_select_cpu, struct task_struct *p, s32 prev_cpu,
 	if (cpu >= 0) {
 		if (can_direct_dispatch(cpu)) {
 			scx_bpf_dsq_insert_vtime(p, cpu_to_dsq(cpu),
-						 SCX_SLICE_DFL, p->scx.dsq_vtime, 0);
+						 slice_ns, p->scx.dsq_vtime, 0);
 			__sync_fetch_and_add(&nr_kernel_dispatches, 1);
 		}
 		return cpu;
@@ -695,7 +700,7 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
 	 * scheduling action to do.
 	 */
 	if (is_usersched_task(p)) {
-		scx_bpf_dsq_insert(p, SCHED_DSQ, SCX_SLICE_DFL, enq_flags);
+		scx_bpf_dsq_insert(p, SCHED_DSQ, slice_ns, enq_flags);
 		goto out_kick;
 	}
 
@@ -708,7 +713,7 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
 	 */
 	if ((is_kthread(p) && p->nr_cpus_allowed == 1) || is_kswapd(p) || is_khugepaged(p)) {
 		scx_bpf_dsq_insert_vtime(p, cpu_to_dsq(prev_cpu),
-					 SCX_SLICE_DFL, p->scx.dsq_vtime, enq_flags);
+					 slice_ns, p->scx.dsq_vtime, enq_flags);
 		__sync_fetch_and_add(&nr_kernel_dispatches, 1);
 		goto out_kick;
 	}
@@ -738,7 +743,7 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
 
 			if (can_direct_dispatch(cpu)) {
 				scx_bpf_dsq_insert_vtime(p, cpu_to_dsq(cpu),
-							 SCX_SLICE_DFL, p->scx.dsq_vtime, enq_flags);
+							 slice_ns, p->scx.dsq_vtime, enq_flags);
 				__sync_fetch_and_add(&nr_kernel_dispatches, 1);
 				goto out_kick;
 			}
@@ -761,7 +766,7 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
 	if (!task) {
 		sched_congested(p);
 		scx_bpf_dsq_insert_vtime(p, SHARED_DSQ,
-					 SCX_SLICE_DFL, p->scx.dsq_vtime, enq_flags);
+					 slice_ns, p->scx.dsq_vtime, enq_flags);
 		__sync_fetch_and_add(&nr_kernel_dispatches, 1);
 		goto out_kick;
 	}
@@ -845,7 +850,7 @@ void BPF_STRUCT_OPS(rustland_dispatch, s32 cpu, struct task_struct *prev)
 	 */
 	if (prev && is_queued(prev) &&
 	    (!is_usersched_task(prev) || usersched_has_pending_tasks()))
-		prev->scx.slice = SCX_SLICE_DFL;
+		prev->scx.slice = slice_ns;
 }
 
 void BPF_STRUCT_OPS(rustland_runnable, struct task_struct *p, u64 enq_flags)
@@ -922,7 +927,7 @@ void BPF_STRUCT_OPS(rustland_stopping, struct task_struct *p, bool runnable)
 void BPF_STRUCT_OPS(rustland_enable, struct task_struct *p)
 {
 	p->scx.dsq_vtime = 0;
-	p->scx.slice = SCX_SLICE_DFL;
+	p->scx.slice = slice_ns;
 }
 
 /*

rust/scx_rustland_core/assets/bpf/main.bpf.c

@@ -14,11 +14,11 @@ ctrlc = { version = "3.1", features = ["termination"] }
 libbpf-rs = "=0.26.0-beta.1"
 libc = "0.2.175"
 scx_utils = { path = "../../../rust/scx_utils", version = "1.0.22" }
-scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.3.7" }
+scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.4.7" }
 
 [build-dependencies]
 scx_cargo = { path = "../../../rust/scx_cargo", version = "1.0.22" }
-scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.3.7" }
+scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.4.7" }
 
 [features]
 enable_backtrace = []

scheds/rust/scx_rlfifo/Cargo.toml

@@ -116,6 +116,7 @@ impl<'a> Scheduler<'a> {
             false,    // partial (false = include all tasks)
             false,    // debug (false = debug mode off)
             true,     // builtin_idle (true = allow BPF to use idle CPUs if available)
+            SLICE_NS, // default time slice (for tasks automatically dispatched by the backend)
             "rlfifo", // name of the scx ops
         )?;
         Ok(Self { bpf })

scheds/rust/scx_rlfifo/src/main.rs

@@ -20,12 +20,12 @@ serde = { version = "1.0.215", features = ["derive"] }
 scx_stats = { path = "../../../rust/scx_stats", version = "1.0.17" }
 scx_stats_derive = { path = "../../../rust/scx_stats/scx_stats_derive", version = "1.0.17" }
 scx_utils = { path = "../../../rust/scx_utils", version = "1.0.22" }
-scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.3.7" }
+scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.4.7" }
 simplelog = "0.12"
 
 [build-dependencies]
 scx_cargo = { path = "../../../rust/scx_cargo", version = "1.0.22" }
-scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.3.7" }
+scx_rustland_core = { path = "../../../rust/scx_rustland_core", version = "2.4.7" }
 
 [features]
 enable_backtrace = []

scheds/rust/scx_rustland/Cargo.toml

@@ -163,6 +163,9 @@ impl<'a> Scheduler<'a> {
     fn init(opts: &'a Opts, open_object: &'a mut MaybeUninit<OpenObject>) -> Result<Self> {
         let stats_server = StatsServer::new(stats::server_data()).launch()?;
 
+        let slice_ns = opts.slice_us * NSEC_PER_USEC;
+        let slice_ns_min = opts.slice_us_min * NSEC_PER_USEC;
+
         // Low-level BPF connector.
         let bpf = BpfScheduler::init(
             open_object,
@@ -171,6 +174,7 @@ impl<'a> Scheduler<'a> {
             opts.partial,
             opts.verbose,
             true, // Enable built-in idle CPU selection policy
+            slice_ns_min,
             "rustland",
         )?;
 
@@ -189,8 +193,8 @@ impl<'a> Scheduler<'a> {
             tasks: BTreeSet::new(),
             vruntime_now: 0,
             init_page_faults: 0,
-            slice_ns: opts.slice_us * NSEC_PER_USEC,
-            slice_ns_min: opts.slice_us_min * NSEC_PER_USEC,
+            slice_ns,
+            slice_ns_min,
         })
     }