scx_rustland: expose voluntary context switches to the scheduler

Provide the number of voluntary context switches (nvcsw) for each task
to the user-space scheduler.

This extra information can then be used by the scheduler to enhance its
decision-making process when scheduling tasks.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

scheds/rust/scx_rustland/src/bpf.rs

@@ -127,8 +127,9 @@ const SCHED_EXT: i32 = 7;
 pub struct QueuedTask {
     pub pid: i32,              // pid that uniquely identifies a task
     pub cpu: i32,              // CPU where the task is running (-1 = exiting)
-    pub sum_exec_runtime: u64, // Total cpu time */
-    pub weight: u64,           // Task static priority */
+    pub sum_exec_runtime: u64, // Total cpu time
+    pub nvcsw: u64,            // Voluntary context switches
+    pub weight: u64,           // Task static priority
 }
 
 // Task queued for dispatching to the BPF component (see bpf_intf::dispatched_task_ctx).
@@ -159,6 +160,7 @@ impl EnqueuedMessage {
             pid: self.inner.pid,
             cpu: self.inner.cpu,
             sum_exec_runtime: self.inner.sum_exec_runtime,
+            nvcsw: self.inner.nvcsw,
             weight: self.inner.weight,
         }
     }

scheds/rust/scx_rustland/src/bpf/intf.h

@@ -35,6 +35,7 @@ struct queued_task_ctx {
 	s32 pid;
 	s32 cpu; /* CPU where the task is running (-1 = exiting) */
 	u64 sum_exec_runtime; /* Total cpu time */
+	u64 nvcsw; /* Voluntary context switches */
 	u64 weight; /* Task static priority */
 };
 

scheds/rust/scx_rustland/src/bpf/main.bpf.c

@@ -386,6 +386,7 @@ static void get_task_info(struct queued_task_ctx *task,
 		return;
 	}
 	task->sum_exec_runtime = p->se.sum_exec_runtime;
+	task->nvcsw = p->nvcsw;
 	task->weight = p->scx.weight;
 	task->cpu = scx_bpf_task_cpu(p);
 }