From ef89d843d953223a1a506f44c5b45c0c9304a4c3 Mon Sep 17 00:00:00 2001
From: Mark Johnston <markj@FreeBSD.org>
Date: Wed, 18 May 2016 03:55:54 +0000
Subject: [PATCH] Do not acquire the thread lock in hardclock_cnt() unless
 needed.

This function only sets thread flags if a SIGPROF or SIGVTALRM timer
has fired, which is almost never the case.

MFC after:	2 weeks
---
 sys/kern/kern_clock.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/sys/kern/kern_clock.c b/sys/kern/kern_clock.c
index 17b85bcfdacc..e7a7a990703e 100644
--- a/sys/kern/kern_clock.c
+++ b/sys/kern/kern_clock.c
@@ -570,9 +570,11 @@ hardclock_cnt(int cnt, int usermode)
 			flags |= TDF_PROFPEND | TDF_ASTPENDING;
 		PROC_ITIMUNLOCK(p);
 	}
-	thread_lock(td);
-	td->td_flags |= flags;
-	thread_unlock(td);
+	if (flags != 0) {
+		thread_lock(td);
+		td->td_flags |= flags;
+		thread_unlock(td);
+	}
 
 #ifdef	HWPMC_HOOKS
 	if (PMC_CPU_HAS_SAMPLES(PCPU_GET(cpuid)))