diff --git a/arch/x86/tools/mcreboot-smp-x86.sh.in b/arch/x86/tools/mcreboot-smp-x86.sh.in
index 8645f123..68790f95 100644
--- a/arch/x86/tools/mcreboot-smp-x86.sh.in
+++ b/arch/x86/tools/mcreboot-smp-x86.sh.in
@@ -126,6 +126,16 @@ if [ "${irqbalance_used}" == "yes" ]; then
     if ! systemctl stop irqbalance.service 2>/dev/null ; then echo "error: stopping irqbalance" >&2; exit 1; fi;
 fi
 
+# Start mcklogd. Note that McKernel blocks when kmsg buffer is full
+# with '-k 1' until mcklogd unblocks it so starting mcklogd must preceeds
+# booting McKernel
+if [ ${LOGMODE} -ne 0 ]
+then
+# Stop mcklogd which has survived McKernel shutdown because mcstop+release.sh is not used
+	pkill mcklogd    
+	SBINDIR=${SBINDIR} ${SBINDIR}/mcklogd -i ${INTERVAL} -f ${facility}
+fi
+
 # Load IHK if not loaded
 if [ "`lsmod | grep ihk`" == "" ]; then
         if ! insmod ${KMODDIR}/ihk.ko; then echo "error: loading ihk" >&2; exit 1; fi;
@@ -249,12 +259,6 @@ if [ "$enable_mcoverlay" == "yes" ]; then
 		fi
 	done
 fi
-if [ ${LOGMODE} -ne 0 ]
-then
-# mcklogd survives when McKernel isn't shut down by mcstop+release.sh
-	pkill mcklogd    
-	SBINDIR=${SBINDIR} ${SBINDIR}/mcklogd -i ${INTERVAL} -f ${facility}
-fi
 
 # Start irqbalance with CPUs and IRQ for McKernel banned
 if [ "${irqbalance_used}" == "yes" ]; then
diff --git a/kernel/debug.c b/kernel/debug.c
index 1061b6fc..7df6d2e3 100644
--- a/kernel/debug.c
+++ b/kernel/debug.c
@@ -37,6 +37,8 @@ static void kprintf_wait(int len, unsigned long *flags_head, int *slide) {
 		if (head < tail) head += buf_len;
 		if (tail + len > buf_len) adj = buf_len - tail;
 		if (head > tail && head <= tail + len + adj) {
+			/* When proceeding tail (producer pointer) by len would
+			   cross head (consumer pointer) in ring-buffer */
 			if (mode != 1) {
 				*slide = 1;
 				break;
@@ -70,6 +72,9 @@ void kputs(char *buf)
 	
 	memcpy(kmsg_buf.str + kmsg_buf.tail, buf, len);
 	kmsg_buf.tail += len;
+	/* When proceeding tail (producer pointer) by len would
+	   cross head (consumer pointer) in ring-buffer, give up
+	   [head, tail] because the range is overwritten */
 	if (slide == 1) {
 		kmsg_buf.head = kmsg_buf.tail + 1;
 		if (kmsg_buf.head >= kmsg_buf.len) kmsg_buf.head = 0;
@@ -170,6 +175,17 @@ int kprintf(const char *format, ...)
 	return len;
 }
 
+/* mode:
+    0: mcklogd is not running.
+       When kmsg buffer is full, writer doesn't block
+       and overwrites the buffer.
+    1: mcklogd periodically retrieves kmsg.
+       When kmsg buffer is full, writer blocks until 
+       someone retrieves kmsg.
+    2: mcklogd periodically retrieves kmsg.
+       When kmsg buffer is full, writer doesn't block
+       and overwrites the buffer.
+*/
 void kmsg_init(int mode)
 {
 	ihk_mc_spinlock_init(&kmsg_lock);