clockintr: add clockintr_stagger()

clockintr_stagger() formalizes the "staggered start" pattern for
scheduling a common periodic clock interrupt across multiple CPUs.

Right now we implement a staggered start by hand for hardclock(),
statclock(), and schedclock().  I expect we will do it for
profclock(), too.

1 files changed, 16 insertions, 1 deletions

diff --git a/sys/kern/kern_clockintr.c b/sys/kern/kern_clockintr.c
index bcd583c193c..1f1f11a25cb 100644
--- a/sys/kern/kern_clockintr.c
+++ b/sys/kern/kern_clockintr.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kern_clockintr.c,v 1.22 2023/06/15 22:18:06 cheloha Exp $ */
+/* $OpenBSD: kern_clockintr.c,v 1.23 2023/06/18 23:09:35 cheloha Exp $ */
 /*
  * Copyright (c) 2003 Dale Rahn <drahn@openbsd.org>
  * Copyright (c) 2020 Mark Kettenis <kettenis@openbsd.org>
@@ -64,6 +64,7 @@ uint64_t clockintr_nsecuptime(const struct clockintr *);
 void clockintr_schedclock(struct clockintr *, void *);
 void clockintr_schedule(struct clockintr *, uint64_t);
 void clockintr_schedule_locked(struct clockintr *, uint64_t);
+void clockintr_stagger(struct clockintr *, uint64_t, u_int, u_int);
 void clockintr_statclock(struct clockintr *, void *);
 void clockintr_statvar_init(int, uint32_t *, uint32_t *, uint32_t *);
 uint64_t clockqueue_next(const struct clockintr_queue *);
@@ -459,6 +460,20 @@ clockintr_schedule_locked(struct clockintr *cl, uint64_t expiration)
 	SET(cl->cl_flags, CLST_PENDING);
 }
 
+void
+clockintr_stagger(struct clockintr *cl, uint64_t period, u_int n, u_int count)
+{
+	struct clockintr_queue *cq = cl->cl_queue;
+
+	KASSERT(n < count);
+
+	mtx_enter(&cq->cq_mtx);
+	if (ISSET(cl->cl_flags, CLST_PENDING))
+		panic("%s: clock interrupt pending", __func__);
+	cl->cl_expiration = period / count * n;
+	mtx_leave(&cq->cq_mtx);
+}
+
 /*
  * Compute the period (avg) for the given frequency and a range around
  * that period.  The range is [min + 1, min + mask].  The range is used