[PATCH] x86-64: Dynamically adjust machine check interval

Background:
We've found that MCEs (specifically DRAM SBEs) tend to come in bunches,
especially when we are trying really hard to stress the system out. The
current MCE poller uses a static interval which does not care whether it
has or has not found MCEs recently.

Description:
This patch makes the MCE poller adjust the polling interval dynamically.
If we find an MCE, poll 2x faster (down to 10 ms). When we stop finding
MCEs, poll 2x slower (up to check_interval seconds). The check_interval
tunable becomes the max polling interval. The "Machine check events
logged" printk() is rate limited to the check_interval, which should be
identical behavior to the old functionality.

Result:
If you start to take a lot of correctable errors (not exceptions), you
log them faster and more accurately (less chance of overflowing the MCA
registers). If you don't take a lot of errors, you will see no change.

Alternatives:
I considered simply reducing the polling interval to 10 ms immediately
and keeping it there as long as we continue to find errors. This felt a
bit heavy handed, but does perform significantly better for the default
check_interval of 5 minutes (we're using a few seconds when testing for
DRAM errors). I could be convinced to go with this, if anyone felt it
was not too aggressive.

Testing:
I used an error-injecting DIMM to create lots of correctable DRAM errors
and verified that the polling interval accelerates. The printk() only
happens once per check_interval seconds.

Patch:
This patch is against 2.6.21-rc7.

Signed-Off-By: Tim Hockin <[e-mail]>
Signed-off-by: Andi Kleen <[e-mail]>

Documentation/x86_64/machinecheck

···
 
 check_interval
 	How often to poll for corrected machine check errors, in seconds
-	(Note output is hexademical). Default 5 minutes.
+	(Note output is hexademical). Default 5 minutes.  When the poller
+	finds MCEs it triggers an exponential speedup (poll more often) on
+	the polling interval.  When the poller stops finding MCEs, it
+	triggers an exponential backoff (poll less often) on the polling
+	interval. The check_interval variable is both the initial and
+	maximum polling interval.
 
 tolerant
 	Tolerance level. When a machine check exception occurs for a non

arch/x86_64/kernel/mce.c

···
 #endif /* CONFIG_X86_MCE_INTEL */
 
 /*
- * Periodic polling timer for "silent" machine check errors.
+ * Periodic polling timer for "silent" machine check errors.  If the
+ * poller finds an MCE, poll 2x faster.  When the poller finds no more
+ * errors, poll 2x slower (up to check_interval seconds).
  */
 
 static int check_interval = 5 * 60; /* 5 minutes */
+static int next_interval; /* in jiffies */
 static void mcheck_timer(struct work_struct *work);
 static DECLARE_DELAYED_WORK(mcheck_work, mcheck_timer);
 
···
 static void mcheck_timer(struct work_struct *work)
 {
 	on_each_cpu(mcheck_check_cpu, NULL, 1, 1);
-	schedule_delayed_work(&mcheck_work, check_interval * HZ);
 
 	/*
 	 * It's ok to read stale data here for notify_user and
···
 	 * writes.
 	 */
 	if (notify_user && console_logged) {
+		static unsigned long last_print;
+		unsigned long now = jiffies;
+
+		/* if we logged an MCE, reduce the polling interval */
+		next_interval = max(next_interval/2, HZ/100);
 		notify_user = 0;
 		clear_bit(0, &console_logged);
-		printk(KERN_INFO "Machine check events logged\n");
+		if (time_after_eq(now, last_print + (check_interval*HZ))) {
+			last_print = now;
+			printk(KERN_INFO "Machine check events logged\n");
+		}
+	} else {
+		next_interval = min(next_interval*2, check_interval*HZ);
 	}
+
+	schedule_delayed_work(&mcheck_work, next_interval);
 }
 
 
 static __init int periodic_mcheck_init(void)
 { 
-	if (check_interval)
-		schedule_delayed_work(&mcheck_work, check_interval*HZ);
+	next_interval = check_interval * HZ;
+	if (next_interval)
+		schedule_delayed_work(&mcheck_work, next_interval);
 	return 0;
 } 
 __initcall(periodic_mcheck_init);
···
 /* Reinit MCEs after user configuration changes */
 static void mce_restart(void) 
 { 
-	if (check_interval)
+	if (next_interval)
 		cancel_delayed_work(&mcheck_work);
 	/* Timer race is harmless here */
 	on_each_cpu(mce_init, NULL, 1, 1);       
-	if (check_interval)
-		schedule_delayed_work(&mcheck_work, check_interval*HZ);
+	next_interval = check_interval * HZ;
+	if (next_interval)
+		schedule_delayed_work(&mcheck_work, next_interval);
 }
 
 static struct sysdev_class mce_sysclass = {