Commit abd50713 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf: Reimplement frequency driven sampling



There was a bug in the old period code that caused intel_pmu_enable_all()
or native_write_msr_safe() to show up quite high in the profiles.

In staring at that code it made my head hurt, so I rewrote it in a
hopefully simpler fashion. Its now fully symetric between tick and
overflow driven adjustments and uses less data to boot.

The only complication is that it basically wants to do a u128 division.
The code approximates that in a rather simple truncate until it fits
fashion, taking care to balance the terms while truncating.

This version does not generate that sampling artefact.
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Cc: <stable@kernel.org>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent ef12a141
......@@ -498,9 +498,8 @@ struct hw_perf_event {
atomic64_t period_left;
u64 interrupts;
u64 freq_count;
u64 freq_interrupts;
u64 freq_stamp;
u64 freq_time_stamp;
u64 freq_count_stamp;
#endif
};
......
......@@ -1423,14 +1423,83 @@ void perf_event_task_sched_in(struct task_struct *task)
static void perf_log_throttle(struct perf_event *event, int enable);
static void perf_adjust_period(struct perf_event *event, u64 events)
static u64 perf_calculate_period(struct perf_event *event, u64 nsec, u64 count)
{
u64 frequency = event->attr.sample_freq;
u64 sec = NSEC_PER_SEC;
u64 divisor, dividend;
int count_fls, nsec_fls, frequency_fls, sec_fls;
count_fls = fls64(count);
nsec_fls = fls64(nsec);
frequency_fls = fls64(frequency);
sec_fls = 30;
/*
* We got @count in @nsec, with a target of sample_freq HZ
* the target period becomes:
*
* @count * 10^9
* period = -------------------
* @nsec * sample_freq
*
*/
/*
* Reduce accuracy by one bit such that @a and @b converge
* to a similar magnitude.
*/
#define REDUCE_FLS(a, b) \
do { \
if (a##_fls > b##_fls) { \
a >>= 1; \
a##_fls--; \
} else { \
b >>= 1; \
b##_fls--; \
} \
} while (0)
/*
* Reduce accuracy until either term fits in a u64, then proceed with
* the other, so that finally we can do a u64/u64 division.
*/
while (count_fls + sec_fls > 64 && nsec_fls + frequency_fls > 64) {
REDUCE_FLS(nsec, frequency);
REDUCE_FLS(sec, count);
}
if (count_fls + sec_fls > 64) {
divisor = nsec * frequency;
while (count_fls + sec_fls > 64) {
REDUCE_FLS(count, sec);
divisor >>= 1;
}
dividend = count * sec;
} else {
dividend = count * sec;
while (nsec_fls + frequency_fls > 64) {
REDUCE_FLS(nsec, frequency);
dividend >>= 1;
}
divisor = nsec * frequency;
}
return div64_u64(dividend, divisor);
}
static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
{
struct hw_perf_event *hwc = &event->hw;
u64 period, sample_period;
s64 delta;
events *= hwc->sample_period;
period = div64_u64(events, event->attr.sample_freq);
period = perf_calculate_period(event, nsec, count);
delta = (s64)(period - hwc->sample_period);
delta = (delta + 7) / 8; /* low pass filter */
......@@ -1441,13 +1510,22 @@ static void perf_adjust_period(struct perf_event *event, u64 events)
sample_period = 1;
hwc->sample_period = sample_period;
if (atomic64_read(&hwc->period_left) > 8*sample_period) {
perf_disable();
event->pmu->disable(event);
atomic64_set(&hwc->period_left, 0);
event->pmu->enable(event);
perf_enable();
}
}
static void perf_ctx_adjust_freq(struct perf_event_context *ctx)
{
struct perf_event *event;
struct hw_perf_event *hwc;
u64 interrupts, freq;
u64 interrupts, now;
s64 delta;
raw_spin_lock(&ctx->lock);
list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
......@@ -1468,44 +1546,18 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx)
if (interrupts == MAX_INTERRUPTS) {
perf_log_throttle(event, 1);
event->pmu->unthrottle(event);
interrupts = 2*sysctl_perf_event_sample_rate/HZ;
}
if (!event->attr.freq || !event->attr.sample_freq)
continue;
/*
* if the specified freq < HZ then we need to skip ticks
*/
if (event->attr.sample_freq < HZ) {
freq = event->attr.sample_freq;
hwc->freq_count += freq;
hwc->freq_interrupts += interrupts;
if (hwc->freq_count < HZ)
continue;
interrupts = hwc->freq_interrupts;
hwc->freq_interrupts = 0;
hwc->freq_count -= HZ;
} else
freq = HZ;
perf_adjust_period(event, freq * interrupts);
event->pmu->read(event);
now = atomic64_read(&event->count);
delta = now - hwc->freq_count_stamp;
hwc->freq_count_stamp = now;
/*
* In order to avoid being stalled by an (accidental) huge
* sample period, force reset the sample period if we didn't
* get any events in this freq period.
*/
if (!interrupts) {
perf_disable();
event->pmu->disable(event);
atomic64_set(&hwc->period_left, 0);
event->pmu->enable(event);
perf_enable();
}
if (delta > 0)
perf_adjust_period(event, TICK_NSEC, delta);
}
raw_spin_unlock(&ctx->lock);
}
......@@ -3768,12 +3820,12 @@ static int __perf_event_overflow(struct perf_event *event, int nmi,
if (event->attr.freq) {
u64 now = perf_clock();
s64 delta = now - hwc->freq_stamp;
s64 delta = now - hwc->freq_time_stamp;
hwc->freq_stamp = now;
hwc->freq_time_stamp = now;
if (delta > 0 && delta < TICK_NSEC)
perf_adjust_period(event, NSEC_PER_SEC / (int)delta);
if (delta > 0 && delta < 2*TICK_NSEC)
perf_adjust_period(event, delta, hwc->last_period);
}
/*
......
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