scx_rustland_core: pass nvcsw, slice and dsq_vtime to user-space

Provide additional task metrics to user-space schedulers via QueuedTask:
 - nvcsw: total amount of voluntary context switches
 - slice: task time slice "budget" (from p->scx.slice)
 - dsq_vtime: current task vtime (from p->scx.dsq_vtime)

In this way user-space schedulers can quickly access these metrics to
implement better scheduling policy.

Signed-off-by: Andrea Righi <andrea.righi@linux.dev>

rust/scx_rustland_core/assets/bpf.rs

@@ -82,7 +82,10 @@ pub struct QueuedTask {
     pub cpu: i32,              // CPU where the task is running
     pub flags: u64,            // task enqueue flags
     pub sum_exec_runtime: u64, // Total cpu time
+    pub nvcsw: u64,            // Total amount of voluntary context switches
     pub weight: u64,           // Task static priority
+    pub slice: u64,            // Time slice budget
+    pub vtime: u64,            // Current vruntime
     cpumask_cnt: u64,          // cpumask generation counter (private)
 }
 
@@ -107,9 +110,9 @@ impl DispatchedTask {
             pid: task.pid,
             cpu: task.cpu,
             flags: task.flags,
-            cpumask_cnt: task.cpumask_cnt,
             slice_ns: 0, // use default time slice
             vtime: 0,
+            cpumask_cnt: task.cpumask_cnt,
         }
     }
 }
@@ -144,7 +147,10 @@ impl EnqueuedMessage {
             cpu: self.inner.cpu,
             flags: self.inner.flags,
             sum_exec_runtime: self.inner.sum_exec_runtime,
+            nvcsw: self.inner.nvcsw,
             weight: self.inner.weight,
+            slice: self.inner.slice,
+            vtime: self.inner.vtime,
             cpumask_cnt: self.inner.cpumask_cnt,
         }
     }

rust/scx_rustland_core/assets/bpf/intf.h

@@ -83,9 +83,12 @@ struct queued_task_ctx {
 	s32 pid;
 	s32 cpu; /* CPU where the task is running */
 	u64 flags; /* task enqueue flags */
-	u64 cpumask_cnt; /* cpumask generation counter */
 	u64 sum_exec_runtime; /* Total cpu time */
+	u64 nvcsw; /* Total amount of voluntary context switches */
 	u64 weight; /* Task static priority */
+	u64 slice; /* Time slice budget */
+	u64 vtime; /* Current task's vruntime */
+	u64 cpumask_cnt; /* cpumask generation counter */
 };
 
 /*

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

@@ -690,6 +690,9 @@ static void dispatch_task(const struct dispatched_task_ctx *task)
 	if (!p)
 		return;
 
+	dbg_msg("dispatch: pid=%d (%s) cpu=0x%lx vtime=%llu slice=%llu",
+		p->pid, p->comm, task->cpu, task->vtime, task->slice_ns);
+
 	/*
 	 * Update task's time slice in its context.
 	 */
@@ -808,10 +811,13 @@ static void get_task_info(struct queued_task_ctx *task,
 	struct task_ctx *tctx = try_lookup_task_ctx(p);
 
 	task->pid = p->pid;
-	task->sum_exec_runtime = p->se.sum_exec_runtime;
-	task->flags = enq_flags;
-	task->weight = p->scx.weight;
 	task->cpu = scx_bpf_task_cpu(p);
+	task->flags = enq_flags;
+	task->sum_exec_runtime = p->se.sum_exec_runtime;
+	task->nvcsw = p->nvcsw;
+	task->weight = p->scx.weight;
+	task->slice = p->scx.slice;
+	task->vtime = p->scx.dsq_vtime;
 	task->cpumask_cnt = tctx ? tctx->cpumask_cnt : 0;
 }
 
@@ -903,6 +909,7 @@ static bool dispatch_user_scheduler(void)
 		scx_bpf_error("Failed to find usersched task %d", usersched_pid);
 		return false;
 	}
+
 	/*
 	 * Use the highest vtime possible to give the scheduler itself the
 	 * lowest priority possible.
@@ -997,6 +1004,11 @@ void BPF_STRUCT_OPS(rustland_dispatch, s32 cpu, struct task_struct *prev)
 	 */
 	bpf_user_ringbuf_drain(&dispatched, handle_dispatched_task, NULL, 0);
 
+	/*
+	 * Check if the user-space scheduler needs to run.
+	 */
+	dispatch_user_scheduler();
+
 	/*
 	 * Try to steal a task dispatched to CPUs that may have gone offline
 	 * (this allows to prevent indefinite task stalls).
@@ -1013,13 +1025,7 @@ void BPF_STRUCT_OPS(rustland_dispatch, s32 cpu, struct task_struct *prev)
 	/*
 	 * Consume a task from the shared DSQ.
 	 */
-	if (scx_bpf_consume(SHARED_DSQ))
+	scx_bpf_consume(SHARED_DSQ);
-		return;
-
-	/*
-	 * Check if the user-space scheduler needs to run.
-	 */
-	dispatch_user_scheduler();
 }
 
 /*