[1/3] genirq/timings: Remove variance computation code

diff --git a/kernel/irq/timings.c b/kernel/irq/timings.c
index 1e4cb63a5c82..3cde046a2bc8 100644
--- a/kernel/irq/timings.c
+++ b/kernel/irq/timings.c
@@ -8,7 +8,6 @@ 
 #include <linux/interrupt.h>
 #include <linux/idr.h>
 #include <linux/irq.h>
-#include <linux/math64.h>
 
 #include <trace/events/irq.h>
 
@@ -19,13 +18,7 @@  DEFINE_STATIC_KEY_FALSE(irq_timing_enabled);
 DEFINE_PER_CPU(struct irq_timings, irq_timings);
 
 struct irqt_stat {
-	u64	next_evt;
-	u64	last_ts;
-	u64	variance;
-	u32	avg;
-	u32	nr_samples;
-	int	anomalies;
-	int	valid;
+	u64     next_evt;
 };
 
 static DEFINE_IDR(irqt_stats);
@@ -40,184 +33,6 @@  void irq_timings_disable(void)
 	static_branch_disable(&irq_timing_enabled);
 }
 
-/**
- * irqs_update - update the irq timing statistics with a new timestamp
- *
- * @irqs: an irqt_stat struct pointer
- * @ts: the new timestamp
- *
- * The statistics are computed online, in other words, the code is
- * designed to compute the statistics on a stream of values rather
- * than doing multiple passes on the values to compute the average,
- * then the variance. The integer division introduces a loss of
- * precision but with an acceptable error margin regarding the results
- * we would have with the double floating precision: we are dealing
- * with nanosec, so big numbers, consequently the mantisse is
- * negligeable, especially when converting the time in usec
- * afterwards.
- *
- * The computation happens at idle time. When the CPU is not idle, the
- * interrupts' timestamps are stored in the circular buffer, when the
- * CPU goes idle and this routine is called, all the buffer's values
- * are injected in the statistical model continuying to extend the
- * statistics from the previous busy-idle cycle.
- *
- * The observations showed a device will trigger a burst of periodic
- * interrupts followed by one or two peaks of longer time, for
- * instance when a SD card device flushes its cache, then the periodic
- * intervals occur again. A one second inactivity period resets the
- * stats, that gives us the certitude the statistical values won't
- * exceed 1x10^9, thus the computation won't overflow.
- *
- * Basically, the purpose of the algorithm is to watch the periodic
- * interrupts and eliminate the peaks.
- *
- * An interrupt is considered periodically stable if the interval of
- * its occurences follow the normal distribution, thus the values
- * comply with:
- *
- *      avg - 3 x stddev < value < avg + 3 x stddev
- *
- * Which can be simplified to:
- *
- *      -3 x stddev < value - avg < 3 x stddev
- *
- *      abs(value - avg) < 3 x stddev
- *
- * In order to save a costly square root computation, we use the
- * variance. For the record, stddev = sqrt(variance). The equation
- * above becomes:
- *
- *      abs(value - avg) < 3 x sqrt(variance)
- *
- * And finally we square it:
- *
- *      (value - avg) ^ 2 < (3 x sqrt(variance)) ^ 2
- *
- *      (value - avg) x (value - avg) < 9 x variance
- *
- * Statistically speaking, any values out of this interval is
- * considered as an anomaly and is discarded. However, a normal
- * distribution appears when the number of samples is 30 (it is the
- * rule of thumb in statistics, cf. "30 samples" on Internet). When
- * there are three consecutive anomalies, the statistics are resetted.
- *
- */
-static void irqs_update(struct irqt_stat *irqs, u64 ts)
-{
-	u64 old_ts = irqs->last_ts;
-	u64 variance = 0;
-	u64 interval;
-	s64 diff;
-
-	/*
-	 * The timestamps are absolute time values, we need to compute
-	 * the timing interval between two interrupts.
-	 */
-	irqs->last_ts = ts;
-
-	/*
-	 * The interval type is u64 in order to deal with the same
-	 * type in our computation, that prevent mindfuck issues with
-	 * overflow, sign and division.
-	 */
-	interval = ts - old_ts;
-
-	/*
-	 * The interrupt triggered more than one second apart, that
-	 * ends the sequence as predictible for our purpose. In this
-	 * case, assume we have the beginning of a sequence and the
-	 * timestamp is the first value. As it is impossible to
-	 * predict anything at this point, return.
-	 *
-	 * Note the first timestamp of the sequence will always fall
-	 * in this test because the old_ts is zero. That is what we
-	 * want as we need another timestamp to compute an interval.
-	 */
-	if (interval >= NSEC_PER_SEC) {
-		memset(irqs, 0, sizeof(*irqs));
-		irqs->last_ts = ts;
-		return;
-	}
-
-	/*
-	 * Pre-compute the delta with the average as the result is
-	 * used several times in this function.
-	 */
-	diff = interval - irqs->avg;
-
-	/*
-	 * Increment the number of samples.
-	 */
-	irqs->nr_samples++;
-
-	/*
-	 * Online variance divided by the number of elements if there
-	 * is more than one sample.  Normally the formula is division
-	 * by nr_samples - 1 but we assume the number of element will be
-	 * more than 32 and dividing by 32 instead of 31 is enough
-	 * precise.
-	 */
-	if (likely(irqs->nr_samples > 1))
-		variance = irqs->variance >> IRQ_TIMINGS_SHIFT;
-
-	/*
-	 * The rule of thumb in statistics for the normal distribution
-	 * is having at least 30 samples in order to have the model to
-	 * apply. Values outside the interval are considered as an
-	 * anomaly.
-	 */
-	if ((irqs->nr_samples >= 30) && ((diff * diff) > (9 * variance))) {
-		/*
-		 * After three consecutive anomalies, we reset the
-		 * stats as it is no longer stable enough.
-		 */
-		if (irqs->anomalies++ >= 3) {
-			memset(irqs, 0, sizeof(*irqs));
-			irqs->last_ts = ts;
-			return;
-		}
-	} else {
-		/*
-		 * The anomalies must be consecutives, so at this
-		 * point, we reset the anomalies counter.
-		 */
-		irqs->anomalies = 0;
-	}
-
-	/*
-	 * The interrupt is considered stable enough to try to predict
-	 * the next event on it.
-	 */
-	irqs->valid = 1;
-
-	/*
-	 * Online average algorithm:
-	 *
-	 *  new_average = average + ((value - average) / count)
-	 *
-	 * The variance computation depends on the new average
-	 * to be computed here first.
-	 *
-	 */
-	irqs->avg = irqs->avg + (diff >> IRQ_TIMINGS_SHIFT);
-
-	/*
-	 * Online variance algorithm:
-	 *
-	 *  new_variance = variance + (value - average) x (value - new_average)
-	 *
-	 * Warning: irqs->avg is updated with the line above, hence
-	 * 'interval - irqs->avg' is no longer equal to 'diff'
-	 */
-	irqs->variance = irqs->variance + (diff * (interval - irqs->avg));
-
-	/*
-	 * Update the next event
-	 */
-	irqs->next_evt = ts + irqs->avg;
-}
-
 /**
  * irq_timings_next_event - Return when the next event is supposed to arrive
  *
@@ -246,12 +61,6 @@  static void irqs_update(struct irqt_stat *irqs, u64 ts)
  */
 u64 irq_timings_next_event(u64 now)
 {
-	struct irq_timings *irqts = this_cpu_ptr(&irq_timings);
-	struct irqt_stat *irqs;
-	struct irqt_stat __percpu *s;
-	u64 ts, next_evt = U64_MAX;
-	int i, irq = 0;
-
 	/*
 	 * This function must be called with the local irq disabled in
 	 * order to prevent the timings circular buffer to be updated
@@ -259,64 +68,7 @@  u64 irq_timings_next_event(u64 now)
 	 */
 	lockdep_assert_irqs_disabled();
 
-	/*
-	 * Number of elements in the circular buffer: If it happens it
-	 * was flushed before, then the number of elements could be
-	 * smaller than IRQ_TIMINGS_SIZE, so the count is used,
-	 * otherwise the array size is used as we wrapped. The index
-	 * begins from zero when we did not wrap. That could be done
-	 * in a nicer way with the proper circular array structure
-	 * type but with the cost of extra computation in the
-	 * interrupt handler hot path. We choose efficiency.
-	 *
-	 * Inject measured irq/timestamp to the statistical model
-	 * while decrementing the counter because we consume the data
-	 * from our circular buffer.
-	 */
-	for (i = irqts->count & IRQ_TIMINGS_MASK,
-		     irqts->count = min(IRQ_TIMINGS_SIZE, irqts->count);
-	     irqts->count > 0; irqts->count--, i = (i + 1) & IRQ_TIMINGS_MASK) {
-
-		irq = irq_timing_decode(irqts->values[i], &ts);
-
-		s = idr_find(&irqt_stats, irq);
-		if (s) {
-			irqs = this_cpu_ptr(s);
-			irqs_update(irqs, ts);
-		}
-	}
-
-	/*
-	 * Look in the list of interrupts' statistics, the earliest
-	 * next event.
-	 */
-	idr_for_each_entry(&irqt_stats, s, i) {
-
-		irqs = this_cpu_ptr(s);
-
-		if (!irqs->valid)
-			continue;
-
-		if (irqs->next_evt <= now) {
-			irq = i;
-			next_evt = now;
-
-			/*
-			 * This interrupt mustn't use in the future
-			 * until new events occur and update the
-			 * statistics.
-			 */
-			irqs->valid = 0;
-			break;
-		}
-
-		if (irqs->next_evt < next_evt) {
-			irq = i;
-			next_evt = irqs->next_evt;
-		}
-	}
-
-	return next_evt;
+	return 0;
 }
 
 void irq_timings_free(int irq)