From bb4ae18c2a7cec1fd8c3877a324a361b7f4cba72 Mon Sep 17 00:00:00 2001
From: Matthew Dillon <dillon@apollo.backplane.com>
Date: Wed, 7 Dec 2011 18:42:01 -0800
Subject: [PATCH] kernel - add DEBUG_LOCKS_LATENCY option

* This is for debugging only, default disabled.  If optioned this adds
  a sysctl to add a forced latency loop (count to N) in front of any
  spinlock or gettoken.
---
 sys/conf/options         |  1 +
 sys/kern/kern_spinlock.c | 16 ++++++++++++++++
 sys/kern/lwkt_token.c    | 26 ++++++++++++++++++++++++--
 3 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/sys/conf/options b/sys/conf/options
index f161292bb2..ca1eb9884c 100644
--- a/sys/conf/options
+++ b/sys/conf/options
@@ -469,6 +469,7 @@ ENABLE_ALART		opt_intpm.h
 BLKDEV_IOSIZE		opt_global.h
 DEBUG			opt_global.h
 DEBUG_LOCKS		opt_global.h
+DEBUG_LOCKS_LATENCY	opt_global.h
 DEBUG_CRIT_SECTIONS	opt_global.h
 DIAGNOSTIC		opt_global.h
 INVARIANTS		opt_global.h
diff --git a/sys/kern/kern_spinlock.c b/sys/kern/kern_spinlock.c
index d836833514..38b222092c 100644
--- a/sys/kern/kern_spinlock.c
+++ b/sys/kern/kern_spinlock.c
@@ -94,6 +94,16 @@ SYSCTL_QUAD(_debug, OID_AUTO, spinlocks_contested2, CTLFLAG_RD,
     &spinlocks_contested2, 0,
     "Serious spinlock contention count");
 
+#ifdef DEBUG_LOCKS_LATENCY
+
+static long spinlocks_add_latency;
+SYSCTL_LONG(_debug, OID_AUTO, spinlocks_add_latency, CTLFLAG_RW,
+    &spinlocks_add_latency, 0,
+    "Add spinlock latency");
+
+#endif
+
+
 /*
  * We need a fairly large pool to avoid contention on large SMP systems,
  * particularly multi-chip systems.
@@ -167,6 +177,12 @@ spin_lock_contested(struct spinlock *spin)
 	struct indefinite_info info = { 0, 0 };
 	int i;
 
+#ifdef DEBUG_LOCKS_LATENCY
+	long j;
+	for (j = spinlocks_add_latency; j > 0; --j)
+		cpu_ccfence();
+#endif
+
 	i = 0;
 	++spin->countb;
 
diff --git a/sys/kern/lwkt_token.c b/sys/kern/lwkt_token.c
index 90c6e8b572..31494505cc 100644
--- a/sys/kern/lwkt_token.c
+++ b/sys/kern/lwkt_token.c
@@ -163,6 +163,16 @@ SYSCTL_LONG(_lwkt, OID_AUTO, tty_collisions, CTLFLAG_RW,
 SYSCTL_LONG(_lwkt, OID_AUTO, vnode_collisions, CTLFLAG_RW,
     &vnode_token.t_collisions, 0, "Collision counter of vnode_token");
 
+#ifdef DEBUG_LOCKS_LATENCY
+
+static long tokens_add_latency;
+SYSCTL_LONG(_debug, OID_AUTO, tokens_add_latency, CTLFLAG_RW,
+	    &tokens_add_latency, 0,
+	    "Add spinlock latency");
+
+#endif
+
+
 static int _lwkt_getalltokens_sorted(thread_t td);
 
 #ifdef SMP
@@ -344,15 +354,27 @@ _lwkt_trytokref_spin(lwkt_tokref_t ref, thread_t td, long mode)
 {
 	int spin;
 
-	if (_lwkt_trytokref(ref, td, mode))
+	if (_lwkt_trytokref(ref, td, mode)) {
+#ifdef DEBUG_LOCKS_LATENCY
+		long j;
+		for (j = tokens_add_latency; j > 0; --j)
+			cpu_ccfence();
+#endif
 		return TRUE;
+	}
 	for (spin = lwkt_token_spin; spin > 0; --spin) {
 		if (lwkt_token_delay)
 			tsc_delay(lwkt_token_delay);
 		else
 			cpu_pause();
-		if (_lwkt_trytokref(ref, td, mode))
+		if (_lwkt_trytokref(ref, td, mode)) {
+#ifdef DEBUG_LOCKS_LATENCY
+			long j;
+			for (j = tokens_add_latency; j > 0; --j)
+				cpu_ccfence();
+#endif
 			return TRUE;
+		}
 	}
 	return FALSE;
 }
-- 
2.41.0