Tech,

Attached is a patch to make the scheduler's round robin tick more event-driven.
- The tick is stopped when the CPU goes idle, rather than firing continuously.
- When scheduling a new process, arm the tick for a full quantum rather than run the tick as a background metronome.

This change primarily affects systems with high CPU counts and frequent idle periods. The intent is to minimize timer interrupts, ensure quantum enforcement when needed, and reduce scheduler overhead on larger SMP/VM systems.

Feedback appreciated.

—
Tim Leslie

diff --git a/sys/kern/kern_sched.c b/sys/kern/kern_sched.c
--- a/sys/kern/kern_sched.c
+++ b/sys/kern/kern_sched.c
@@ -35,6 +35,8 @@ void sched_kthreads_create(void *);
 int sched_proc_to_cpu_cost(struct cpu_info *ci, struct proc *p);
 struct proc *sched_steal_proc(struct cpu_info *);
 
+uint64_t roundrobinperiod; /* round robin period in ns */
+
 /*
  * To help choosing which cpu should run which process we keep track
  * of cpus which are currently idle and which cpus have processes
@@ -358,8 +360,10 @@ again:
 		sched_noidle++;
 		if (p->p_stat != SRUN)
 			panic("thread %d not in SRUN: %d", p->p_tid, p->p_stat);
+		clockintr_advance(&spc->spc_roundrobin, roundrobin_period);
 	} else if ((p = sched_steal_proc(curcpu())) == NULL) {
 		p = spc->spc_idleproc;
+		clockintr_cancel(&spc->spc_roundrobin);
 		if (p == NULL)
 			panic("no idleproc set on CPU%d",
 			    CPU_INFO_UNIT(curcpu()));
@@ -538,6 +542,7 @@ sched_steal_proc(struct cpu_info *self)
 
 	remrunqueue(best);
 	best->p_cpu = self;
+	clockintr_advance(&spc->spc_roundrobin, roundrobin_period);
 
 	sched_stolen++;
 #endif