From 317da70593f68e03d6414e480c11f283f9feb253 Mon Sep 17 00:00:00 2001
From: Jeff Roberson <jeff@FreeBSD.org>
Date: Wed, 23 Jan 2008 03:10:18 +0000
Subject: [PATCH]  - sched_prio() should only adjust tdq_lowpri if the thread
 is running or on    a run-queue.  If the priority is numerically raised only
 change lowpri    if we're certain it will be correct.  Some slop is allowed
 however    previously we could erroneously raise lowpri for an idle cpu that
 a    thread had recently run on which lead to errors in load balancing   
 decisions.

---
 sys/kern/sched_ule.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c
index 9957a25c32f..706adf94eb9 100644
--- a/sys/kern/sched_ule.c
+++ b/sys/kern/sched_ule.c
@@ -1617,16 +1617,18 @@ sched_thread_priority(struct thread *td, u_char prio)
 		sched_rem(td);
 		td->td_priority = prio;
 		sched_add(td, SRQ_BORROWING);
-	} else {
 #ifdef SMP
+	} else if (TD_IS_RUNNING(td)) {
 		struct tdq *tdq;
 
 		tdq = TDQ_CPU(ts->ts_cpu);
-		if (prio < tdq->tdq_lowpri)
+		if (prio < tdq->tdq_lowpri ||
+		   (td->td_priority == tdq->tdq_lowpri && tdq->tdq_load <= 1))
 			tdq->tdq_lowpri = prio;
-#endif
 		td->td_priority = prio;
-	}
+#endif
+	} else
+		td->td_priority = prio;
 }
 
 /*
@@ -1843,8 +1845,6 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
 	mtx = td->td_lock;
 #ifdef SMP
 	ts->ts_rltick = ticks;
-	if (newtd && newtd->td_priority < tdq->tdq_lowpri)
-		tdq->tdq_lowpri = newtd->td_priority;
 #endif
 	td->td_lastcpu = td->td_oncpu;
 	td->td_oncpu = NOCPU;
@@ -2265,6 +2265,7 @@ sched_choose(void)
 	struct tdq_group *tdg;
 #endif
 	struct td_sched *ts;
+	struct thread *td;
 	struct tdq *tdq;
 
 	tdq = TDQ_SELF();
@@ -2274,6 +2275,7 @@ sched_choose(void)
 		tdq_runq_rem(tdq, ts);
 		return (ts->ts_thread);
 	}
+	td = PCPU_GET(idlethread);
 #ifdef SMP
 	/*
 	 * We only set the idled bit when all of the cpus in the group are
@@ -2284,9 +2286,9 @@ sched_choose(void)
 	tdg->tdg_idlemask |= PCPU_GET(cpumask);
 	if (tdg->tdg_idlemask == tdg->tdg_cpumask)
 		atomic_set_int(&tdq_idle, tdg->tdg_mask);
-	tdq->tdq_lowpri = PRI_MAX_IDLE;
+	tdq->tdq_lowpri = td->td_priority;
 #endif
-	return (PCPU_GET(idlethread));
+	return (td);
 }
 
 /*