Re: [PATCH V10 3/3] irq: Compute the periodic interval for interrupts
From: Daniel Lezcano
Date: Fri Jun 02 2017 - 04:57:55 EST
On 23/05/2017 14:55, Daniel Lezcano wrote:
> On 16/05/2017 21:44, Daniel Lezcano wrote:
>> An interrupt behaves with a burst of activity with periodic interval of time
>> followed by one or two peaks of longer interval.
>>
>> As the time intervals are periodic, statistically speaking they follow a normal
>> distribution and each interrupts can be tracked individually.
>>
>> This patch does statistics on all interrupts, except the timers which are
>> deterministic by essence. The goal is to extract the periodicity for each
>> interrupt, with the last timestamp and sum them, so we have the next event.
>>
>> Taking the earliest prediction gives the expected wakeup on the system (assuming
>> a timer won't expire before).
>>
>> As stated in the previous patch, this code is not enabled in the kernel by
>> default.
>>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Rafael J. Wysocki <rafael@xxxxxxxxxx>
>> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>> Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
>
> Hi all,
>
> I do believe I have taken into account all the previous comments. Is
> there any objections for these patches?
Hi Thomas,
there is no objection on this series.
Can you consider merging them?
Thanks in advance.
-- Daniel
>> ---
>> Changelog:
>> V10:
>> - Simplified index/count computation
>> V9:
>> - Deal with 48+16 bits encoded values
>> - Changed irq_stat => irqt_stat to prevent name collision on s390
>> - Changed div64 by constant IRQ_TIMINGS_SHIFT bit shift for average
>> - Changed div64 by constant IRQ_TIMINGS_SHIFT bit shift for variance
>> ---
>> include/linux/interrupt.h | 1 +
>> kernel/irq/internals.h | 21 ++-
>> kernel/irq/timings.c | 336 ++++++++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 357 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
>> index 4c9d3ca..1546ccd 100644
>> --- a/include/linux/interrupt.h
>> +++ b/include/linux/interrupt.h
>> @@ -715,6 +715,7 @@ 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;
>> diff --git a/kernel/irq/internals.h b/kernel/irq/internals.h
>> index df51b5e0..bf3d827 100644
>> --- a/kernel/irq/internals.h
>> +++ b/kernel/irq/internals.h
>> @@ -237,18 +237,26 @@ irq_pm_remove_action(struct irq_desc *desc, struct irqaction *action) { }
>>
>> struct irq_timings {
>> u64 values[IRQ_TIMINGS_SIZE]; /* our circular buffer */
>> - unsigned int count; /* Number of interruptions since last inspection */
>> + int count; /* Number of interruptions since last inspection */
>> };
>>
>> 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 remove_timings(struct irq_desc *desc)
>> {
>> desc->istate &= ~IRQS_TIMINGS;
>> +
>> + irq_timings_free(irq_desc_get_irq(desc));
>> }
>>
>> static inline void 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.
>> @@ -256,6 +264,17 @@ static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)
>> 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;
>> }
>>
>> diff --git a/kernel/irq/timings.c b/kernel/irq/timings.c
>> index 56cf687..4af7703 100644
>> --- a/kernel/irq/timings.c
>> +++ b/kernel/irq/timings.c
>> @@ -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 ne; /* next event */
>> + u64 lts; /* last timestamp */
>> + u64 variance; /* variance */
>> + u32 avg; /* mean value */
>> + u32 count; /* number of samples */
>> + int anomalies; /* number of consecutives anomalies */
>> + int valid; /* behaviour of the interrupt */
>> +};
>> +
>> +static DEFINE_IDR(irqt_stats);
>> +
>> void irq_timings_enable(void)
>> {
>> static_branch_enable(&irq_timing_enabled);
>> @@ -28,3 +46,321 @@ 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
>> + *
>> + * ** This function must be called with the local irq disabled **
>> + *
>> + * 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->lts;
>> + u64 variance = 0;
>> + u64 interval;
>> + s64 diff;
>> +
>> + /*
>> + * The timestamps are absolute time values, we need to compute
>> + * the timing interval between two interrupts.
>> + */
>> + irqs->lts = 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->lts = 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->count++;
>> +
>> + /*
>> + * Online variance divided by the number of elements if there
>> + * is more than one sample. Normally the formula is division
>> + * by count - 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->count > 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->count >= 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->lts = 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->ne = ts + irqs->avg;
>> +}
>> +
>> +/**
>> + * irq_timings_next_event - Return when the next event is supposed to arrive
>> + *
>> + * *** This function must be called with the local irq disabled ***
>> + *
>> + * 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, ne = U64_MAX;
>> + int i, irq = 0;
>> +
>> + /*
>> + * 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_timing_decode(irqts->values[i], &ts, &irq);
>> +
>> + 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->ne <= now) {
>> + irq = i;
>> + ne = now;
>> +
>> + /*
>> + * This interrupt mustn't use in the future
>> + * until new events occur and update the
>> + * statistics.
>> + */
>> + irqs->valid = 0;
>> + break;
>> + }
>> +
>> + if (irqs->ne < ne) {
>> + irq = i;
>> + ne = irqs->ne;
>> + }
>> + }
>> +
>> + return ne;
>> +}
>> +
>> +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)
>> +{
>> + int id;
>> + struct irqt_stat __percpu *s;
>> +
>> + /*
>> + * 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;
>> +}
>>
>
>
--
<http://www.linaro.org/> Linaro.org â Open source software for ARM SoCs
Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog