1 files changed, 369 insertions, 0 deletions

diff --git a/kernel/irq/timings.c b/kernel/irq/timings.c
new file mode 100644
index 000000000000..c8c1d073fbf1
--- /dev/null
+++ b/kernel/irq/timings.c
@@ -0,0 +1,369 @@
+/*
+ * linux/kernel/irq/timings.c
+ *
+ * Copyright (C) 2016, Linaro Ltd - Daniel Lezcano <daniel.lezcano@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ */
+#include <linux/kernel.h>
+#include <linux/percpu.h>
+#include <linux/slab.h>
+#include <linux/static_key.h>
+#include <linux/interrupt.h>
+#include <linux/idr.h>
+#include <linux/irq.h>
+#include <linux/math64.h>
+
+#include <trace/events/irq.h>
+
+#include "internals.h"
+
+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;
+};
+
+static DEFINE_IDR(irqt_stats);
+
+void irq_timings_enable(void)
+{
+        static_branch_enable(&irq_timing_enabled);
+}
+
+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
+ *
+ * During the last busy cycle, the number of interrupts is incremented
+ * and stored in the irq_timings structure. This information is
+ * necessary to:
+ *
+ * - know if the index in the table wrapped up:
+ *
+ *      If more than the array size interrupts happened during the
+ *      last busy/idle cycle, the index wrapped up and we have to
+ *      begin with the next element in the array which is the last one
+ *      in the sequence, otherwise it is a the index 0.
+ *
+ * - have an indication of the interrupts activity on this CPU
+ *   (eg. irq/sec)
+ *
+ * The values are 'consumed' after inserting in the statistical model,
+ * thus the count is reinitialized.
+ *
+ * The array of values **must** be browsed in the time direction, the
+ * timestamp must increase between an element and the next one.
+ *
+ * Returns a nanosec time based estimation of the earliest interrupt,
+ * U64_MAX otherwise.
+ */
+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
+         * while we are reading it.
+         */
+        WARN_ON_ONCE(!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;
+}
+
+void irq_timings_free(int irq)
+{
+        struct irqt_stat __percpu *s;
+
+        s = idr_find(&irqt_stats, irq);
+        if (s) {
+                free_percpu(s);
+                idr_remove(&irqt_stats, irq);
+        }
+}
+
+int irq_timings_alloc(int irq)
+{
+        struct irqt_stat __percpu *s;
+        int id;
+
+        /*
+         * Some platforms can have the same private interrupt per cpu,
+         * so this function may be be called several times with the
+         * same interrupt number. Just bail out in case the per cpu
+         * stat structure is already allocated.
+         */
+        s = idr_find(&irqt_stats, irq);
+        if (s)
+                return 0;
+
+        s = alloc_percpu(*s);
+        if (!s)
+                return -ENOMEM;
+
+        idr_preload(GFP_KERNEL);
+        id = idr_alloc(&irqt_stats, s, irq, irq + 1, GFP_NOWAIT);
+        idr_preload_end();
+
+        if (id < 0) {
+                free_percpu(s);
+                return id;
+        }
+
+        return 0;
+}