From 32aef9ff05d6dda84bd8924efca2f5fb30e602d2 Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Fri, 17 Nov 2017 02:45:38 +0000
Subject: [PATCH] sched: move panic handling code out of choosethread

This avoids jumps in the common case of the kernel not being panicked.
---
 sys/kern/kern_switch.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/sys/kern/kern_switch.c b/sys/kern/kern_switch.c
index f58acd5ed4c6..2cdec8d930f9 100644
--- a/sys/kern/kern_switch.c
+++ b/sys/kern/kern_switch.c
@@ -150,22 +150,21 @@ SYSCTL_PROC(_kern_sched_stats, OID_AUTO, reset, CTLTYPE_INT | CTLFLAG_WR, NULL,
 /*
  * Select the thread that will be run next.
  */
-struct thread *
-choosethread(void)
-{
-	struct thread *td;
 
-retry:
-	td = sched_choose();
+static __noinline struct thread *
+choosethread_panic(struct thread *td)
+{
 
 	/*
 	 * If we are in panic, only allow system threads,
 	 * plus the one we are running in, to be run.
 	 */
-	if (panicstr && ((td->td_proc->p_flag & P_SYSTEM) == 0 &&
+retry:
+	if (((td->td_proc->p_flag & P_SYSTEM) == 0 &&
 	    (td->td_flags & TDF_INPANIC) == 0)) {
 		/* note that it is no longer on the run queue */
 		TD_SET_CAN_RUN(td);
+		td = sched_choose();
 		goto retry;
 	}
 
@@ -173,6 +172,20 @@ retry:
 	return (td);
 }
 
+struct thread *
+choosethread(void)
+{
+	struct thread *td;
+
+	td = sched_choose();
+
+	if (__predict_false(panicstr != NULL))
+		return (choosethread_panic(td));
+
+	TD_SET_RUNNING(td);
+	return (td);
+}
+
 /*
  * Kernel thread preemption implementation.  Critical sections mark
  * regions of code in which preemptions are not allowed.