@@ -706,6 +706,7 @@ static inline void init_irq_proc(void)
#ifdef CONFIG_IRQ_TIMINGS
void irq_timings_enable(void);
void irq_timings_disable(void);
+u64 irq_timings_next_event(u64 now);
#endif
struct seq_file;
@@ -247,13 +247,21 @@ struct irq_timings {
DECLARE_PER_CPU(struct irq_timings, irq_timings);
+extern void irq_timings_free(int irq);
+extern int irq_timings_alloc(int irq);
+
static inline void irq_remove_timings(struct irq_desc *desc)
{
desc->istate &= ~IRQS_TIMINGS;
+
+ irq_timings_free(irq_desc_get_irq(desc));
}
static inline void irq_setup_timings(struct irq_desc *desc, struct irqaction *act)
{
+ int irq = irq_desc_get_irq(desc);
+ int ret;
+
/*
* We don't need the measurement because the idle code already
* knows the next expiry event.
@@ -261,6 +269,17 @@ static inline void irq_setup_timings(struct irq_desc *desc, struct irqaction *ac
if (act->flags & __IRQF_TIMER)
return;
+ /*
+ * In case the timing allocation fails, we just want to warn,
+ * not fail, so letting the system boot anyway.
+ */
+ ret = irq_timings_alloc(irq);
+ if (ret) {
+ pr_warn("Failed to allocate irq timing stats for irq%d (%d)",
+ irq, ret);
+ return;
+ }
+
desc->istate |= IRQS_TIMINGS;
}
@@ -8,10 +8,16 @@
* 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"
@@ -19,6 +25,18 @@ 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);
@@ -28,3 +46,324 @@ 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;
+}