Index: FreeBSD/sys/x86/x86/mca.c
===================================================================
--- FreeBSD/sys/x86/x86/mca.c
+++ FreeBSD/sys/x86/x86/mca.c
@@ -127,6 +127,7 @@
 static struct cmc_state **cmc_state;		/* Indexed by cpuid, bank. */
 static struct amd_et_state *amd_et_state;	/* Indexed by cpuid. */
 static int cmc_throttle = 60;	/* Time in seconds to throttle CMCI. */
+static int cmc_backoff = 1;	/* Throttling exponential backoff factor. */
 
 static int amd_elvt = -1;
 
@@ -579,12 +580,16 @@
 	 * If an interrupt was received less than cmc_throttle seconds
 	 * since the previous interrupt and the count from the current
 	 * event is greater than or equal to the current threshold,
-	 * double the threshold up to the max.
+	 * exponentially increase the threshold up to the max.
 	 */
 	if (mode == CMCI && valid) {
 		if (delta < cmc_throttle && count >= limit &&
 		    limit < max_threshold) {
-			limit = min(limit << 1, max_threshold);
+			limit <<= cmc_backoff;
+			if (limit < cur_threshold)
+				limit = max_threshold;
+			else
+				limit = min(limit, max_threshold);
 		}
 		return (limit);
 	}
@@ -835,6 +840,10 @@
 	    "cmc_throttle", CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE,
 	    &cmc_throttle, 0, sysctl_positive_int, "I",
 	    "Interval in seconds to throttle corrected MC interrupts");
+	SYSCTL_ADD_PROC(NULL, SYSCTL_STATIC_CHILDREN(_hw_mca), OID_AUTO,
+	    "cmc_backoff", CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE,
+	    &cmc_backoff, 0, sysctl_positive_int, "I",
+	    "Backoff factor while throttling (2 ** value)");
 }
 
 static void
@@ -847,6 +856,10 @@
 	    "cmc_throttle", CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE,
 	    &cmc_throttle, 0, sysctl_positive_int, "I",
 	    "Interval in seconds to throttle corrected MC interrupts");
+	SYSCTL_ADD_PROC(NULL, SYSCTL_STATIC_CHILDREN(_hw_mca), OID_AUTO,
+	    "cmc_backoff", CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE,
+	    &cmc_backoff, 0, sysctl_positive_int, "I",
+	    "Backoff factor while throttling (2 ** value)");
 }
 #endif