Patch: Make busy-loops in kipmid configurable (take 3)

This patch adds a module parameter "kipmid_max_busy" that specifies how many
microseconds kipmid should busy-loop before going to sleep. The
new parameter defaults to 0 (previous behavior - busy-loop forever).

The main rationale of this patch is to be able to avoid irritating users
who see lots of CPU time spent int kipmid0. With a bit of tuning of the
kipmid_max_busy parameter it is possible to decrease the CPU usage of kipmid
to negligible values without losing much performance.

In this version of the patch, the parameter can be specified per-interface,
like other ipmi_si parameters, and can be modified at runtime via sysfs.

Signed-off-by: martin.wilck@ts.fujitsu.com

--- a/drivers/char/ipmi/ipmi_si_intf.c	2009-03-24 00:12:14.000000000 +0100
+++ b/drivers/char/ipmi/ipmi_si_intf.c	2009-04-06 17:16:41.000000000 +0200
@@ -297,6 +297,9 @@ struct smi_info {
 static int force_kipmid[SI_MAX_PARMS];
 static int num_force_kipmid;
 
+static unsigned int kipmid_max_busy[SI_MAX_PARMS];
+static int num_max_busy;
+
 static int unload_when_empty = 1;
 
 static int try_smi_init(struct smi_info *smi);
@@ -927,23 +930,57 @@ static void set_run_to_completion(void *
 	}
 }
 
+static int ipmi_thread_must_sleep(enum si_sm_result smi_result, int intf_num,
+				  int *busy, struct timespec *busy_time)
+{
+	unsigned int max_busy = 0;
+	if (smi_result != SI_SM_CALL_WITH_DELAY) {
+		if (*busy)
+			*busy = 0;
+		return 0;
+	}
+	if (intf_num < num_max_busy)
+		max_busy = kipmid_max_busy[intf_num];
+	if (*busy == 0) {
+		*busy = 1;
+		getnstimeofday(busy_time);
+		timespec_add_ns(busy_time, max_busy*NSEC_PER_USEC);
+	} else if (max_busy == 0)
+		return 0;
+	else {
+		struct timespec now;
+		getnstimeofday(&now);
+		if (unlikely(timespec_compare(&now, busy_time) > 0)) {
+			*busy = 0;
+			return 1;
+		}
+	}
+	return 0;
+}
+
 static int ipmi_thread(void *data)
 {
 	struct smi_info *smi_info = data;
 	unsigned long flags;
 	enum si_sm_result smi_result;
+	struct timespec busy_time;
+	int busy = 0;
 
 	set_user_nice(current, 19);
 	while (!kthread_should_stop()) {
+		int must_sleep;
 		spin_lock_irqsave(&(smi_info->si_lock), flags);
 		smi_result = smi_event_handler(smi_info, 0);
 		spin_unlock_irqrestore(&(smi_info->si_lock), flags);
+		must_sleep = ipmi_thread_must_sleep(smi_result,
+						    smi_info->intf_num,
+						    &busy, &busy_time);
 		if (smi_result == SI_SM_CALL_WITHOUT_DELAY)
 			; /* do nothing */
-		else if (smi_result == SI_SM_CALL_WITH_DELAY)
+		else if (smi_result == SI_SM_CALL_WITH_DELAY && !must_sleep)
 			schedule();
 		else
-			schedule_timeout_interruptible(1);
+			schedule_timeout_interruptible(0);
 	}
 	return 0;
 }
@@ -1213,6 +1250,11 @@ module_param(unload_when_empty, int, 0);
 MODULE_PARM_DESC(unload_when_empty, "Unload the module if no interfaces are"
 		 " specified or found, default is 1.  Setting to 0"
 		 " is useful for hot add of devices using hotmod.");
+module_param_array(kipmid_max_busy, uint, &num_max_busy, 0644);
+MODULE_PARM_DESC(kipmid_max_busy,
+		 "Max time (in microseconds) to busy-wait for IPMI data before"
+		 " sleeping. 0 (default) means to wait forever. Set to 100-500"
+		 " if kipmid is using up a lot of CPU time.");
 
 
 static void std_irq_cleanup(struct smi_info *info)