Commit 69d25870 authored by Arjan van de Ven's avatar Arjan van de Ven Committed by Linus Torvalds

cpuidle: fix the menu governor to boost IO performance

Fix the menu idle governor which balances power savings, energy efficiency
and performance impact.

The reason for a reworked governor is that there have been serious
performance issues reported with the existing code on Nehalem server
systems.

To show this I'm sure Andrew wants to see benchmark results:
(benchmark is "fio", "no cstates" is using "idle=poll")

		no cstates	current linux	new algorithm
1 disk		107 Mb/s	85 Mb/s		105 Mb/s
2 disks		215 Mb/s	123 Mb/s	209 Mb/s
12 disks	590 Mb/s	320 Mb/s	585 Mb/s

In various power benchmark measurements, no degredation was found by our
measurement&diagnostics team.  Obviously a small percentage more power was
used in the "fio" benchmark, due to the much higher performance.

While it would be a novel idea to describe the new algorithm in this
commit message, I cheaped out and described it in comments in the code
instead.

[changes since first post: spelling fixes from akpm, review feedback,
folded menu-tng into menu.c]
Signed-off-by: default avatarArjan van de Ven <arjan@linux.intel.com>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Acked-by: default avatarIngo Molnar <mingo@elte.hu>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 45d80eea
......@@ -2,8 +2,12 @@
* menu.c - the menu idle governor
*
* Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
* Copyright (C) 2009 Intel Corporation
* Author:
* Arjan van de Ven <arjan@linux.intel.com>
*
* This code is licenced under the GPL.
* This code is licenced under the GPL version 2 as described
* in the COPYING file that acompanies the Linux Kernel.
*/
#include <linux/kernel.h>
......@@ -13,20 +17,153 @@
#include <linux/ktime.h>
#include <linux/hrtimer.h>
#include <linux/tick.h>
#include <linux/sched.h>
#define BREAK_FUZZ 4 /* 4 us */
#define PRED_HISTORY_PCT 50
#define BUCKETS 12
#define RESOLUTION 1024
#define DECAY 4
#define MAX_INTERESTING 50000
/*
* Concepts and ideas behind the menu governor
*
* For the menu governor, there are 3 decision factors for picking a C
* state:
* 1) Energy break even point
* 2) Performance impact
* 3) Latency tolerance (from pmqos infrastructure)
* These these three factors are treated independently.
*
* Energy break even point
* -----------------------
* C state entry and exit have an energy cost, and a certain amount of time in
* the C state is required to actually break even on this cost. CPUIDLE
* provides us this duration in the "target_residency" field. So all that we
* need is a good prediction of how long we'll be idle. Like the traditional
* menu governor, we start with the actual known "next timer event" time.
*
* Since there are other source of wakeups (interrupts for example) than
* the next timer event, this estimation is rather optimistic. To get a
* more realistic estimate, a correction factor is applied to the estimate,
* that is based on historic behavior. For example, if in the past the actual
* duration always was 50% of the next timer tick, the correction factor will
* be 0.5.
*
* menu uses a running average for this correction factor, however it uses a
* set of factors, not just a single factor. This stems from the realization
* that the ratio is dependent on the order of magnitude of the expected
* duration; if we expect 500 milliseconds of idle time the likelihood of
* getting an interrupt very early is much higher than if we expect 50 micro
* seconds of idle time. A second independent factor that has big impact on
* the actual factor is if there is (disk) IO outstanding or not.
* (as a special twist, we consider every sleep longer than 50 milliseconds
* as perfect; there are no power gains for sleeping longer than this)
*
* For these two reasons we keep an array of 12 independent factors, that gets
* indexed based on the magnitude of the expected duration as well as the
* "is IO outstanding" property.
*
* Limiting Performance Impact
* ---------------------------
* C states, especially those with large exit latencies, can have a real
* noticable impact on workloads, which is not acceptable for most sysadmins,
* and in addition, less performance has a power price of its own.
*
* As a general rule of thumb, menu assumes that the following heuristic
* holds:
* The busier the system, the less impact of C states is acceptable
*
* This rule-of-thumb is implemented using a performance-multiplier:
* If the exit latency times the performance multiplier is longer than
* the predicted duration, the C state is not considered a candidate
* for selection due to a too high performance impact. So the higher
* this multiplier is, the longer we need to be idle to pick a deep C
* state, and thus the less likely a busy CPU will hit such a deep
* C state.
*
* Two factors are used in determing this multiplier:
* a value of 10 is added for each point of "per cpu load average" we have.
* a value of 5 points is added for each process that is waiting for
* IO on this CPU.
* (these values are experimentally determined)
*
* The load average factor gives a longer term (few seconds) input to the
* decision, while the iowait value gives a cpu local instantanious input.
* The iowait factor may look low, but realize that this is also already
* represented in the system load average.
*
*/
struct menu_device {
int last_state_idx;
unsigned int expected_us;
unsigned int predicted_us;
unsigned int current_predicted_us;
unsigned int last_measured_us;
unsigned int elapsed_us;
u64 predicted_us;
unsigned int measured_us;
unsigned int exit_us;
unsigned int bucket;
u64 correction_factor[BUCKETS];
};
#define LOAD_INT(x) ((x) >> FSHIFT)
#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
static int get_loadavg(void)
{
unsigned long this = this_cpu_load();
return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
}
static inline int which_bucket(unsigned int duration)
{
int bucket = 0;
/*
* We keep two groups of stats; one with no
* IO pending, one without.
* This allows us to calculate
* E(duration)|iowait
*/
if (nr_iowait_cpu())
bucket = BUCKETS/2;
if (duration < 10)
return bucket;
if (duration < 100)
return bucket + 1;
if (duration < 1000)
return bucket + 2;
if (duration < 10000)
return bucket + 3;
if (duration < 100000)
return bucket + 4;
return bucket + 5;
}
/*
* Return a multiplier for the exit latency that is intended
* to take performance requirements into account.
* The more performance critical we estimate the system
* to be, the higher this multiplier, and thus the higher
* the barrier to go to an expensive C state.
*/
static inline int performance_multiplier(void)
{
int mult = 1;
/* for higher loadavg, we are more reluctant */
mult += 2 * get_loadavg();
/* for IO wait tasks (per cpu!) we add 5x each */
mult += 10 * nr_iowait_cpu();
return mult;
}
static DEFINE_PER_CPU(struct menu_device, menu_devices);
/**
......@@ -38,37 +175,59 @@ static int menu_select(struct cpuidle_device *dev)
struct menu_device *data = &__get_cpu_var(menu_devices);
int latency_req = pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY);
int i;
int multiplier;
data->last_state_idx = 0;
data->exit_us = 0;
/* Special case when user has set very strict latency requirement */
if (unlikely(latency_req == 0)) {
data->last_state_idx = 0;
if (unlikely(latency_req == 0))
return 0;
}
/* determine the expected residency time */
/* determine the expected residency time, round up */
data->expected_us =
(u32) ktime_to_ns(tick_nohz_get_sleep_length()) / 1000;
DIV_ROUND_UP((u32)ktime_to_ns(tick_nohz_get_sleep_length()), 1000);
data->bucket = which_bucket(data->expected_us);
multiplier = performance_multiplier();
/*
* if the correction factor is 0 (eg first time init or cpu hotplug
* etc), we actually want to start out with a unity factor.
*/
if (data->correction_factor[data->bucket] == 0)
data->correction_factor[data->bucket] = RESOLUTION * DECAY;
/* Make sure to round up for half microseconds */
data->predicted_us = DIV_ROUND_CLOSEST(
data->expected_us * data->correction_factor[data->bucket],
RESOLUTION * DECAY);
/*
* We want to default to C1 (hlt), not to busy polling
* unless the timer is happening really really soon.
*/
if (data->expected_us > 5)
data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
/* Recalculate predicted_us based on prediction_history_pct */
data->predicted_us *= PRED_HISTORY_PCT;
data->predicted_us += (100 - PRED_HISTORY_PCT) *
data->current_predicted_us;
data->predicted_us /= 100;
/* find the deepest idle state that satisfies our constraints */
for (i = CPUIDLE_DRIVER_STATE_START + 1; i < dev->state_count; i++) {
for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) {
struct cpuidle_state *s = &dev->states[i];
if (s->target_residency > data->expected_us)
break;
if (s->target_residency > data->predicted_us)
break;
if (s->exit_latency > latency_req)
break;
if (s->exit_latency * multiplier > data->predicted_us)
break;
data->exit_us = s->exit_latency;
data->last_state_idx = i;
}
data->last_state_idx = i - 1;
return i - 1;
return data->last_state_idx;
}
/**
......@@ -85,35 +244,49 @@ static void menu_reflect(struct cpuidle_device *dev)
unsigned int last_idle_us = cpuidle_get_last_residency(dev);
struct cpuidle_state *target = &dev->states[last_idx];
unsigned int measured_us;
u64 new_factor;
/*
* Ugh, this idle state doesn't support residency measurements, so we
* are basically lost in the dark. As a compromise, assume we slept
* for one full standard timer tick. However, be aware that this
* could potentially result in a suboptimal state transition.
* for the whole expected time.
*/
if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
last_idle_us = USEC_PER_SEC / HZ;
last_idle_us = data->expected_us;
measured_us = last_idle_us;
/*
* measured_us and elapsed_us are the cumulative idle time, since the
* last time we were woken out of idle by an interrupt.
* We correct for the exit latency; we are assuming here that the
* exit latency happens after the event that we're interested in.
*/
if (data->elapsed_us <= data->elapsed_us + last_idle_us)
measured_us = data->elapsed_us + last_idle_us;
if (measured_us > data->exit_us)
measured_us -= data->exit_us;
/* update our correction ratio */
new_factor = data->correction_factor[data->bucket]
* (DECAY - 1) / DECAY;
if (data->expected_us > 0 && data->measured_us < MAX_INTERESTING)
new_factor += RESOLUTION * measured_us / data->expected_us;
else
measured_us = -1;
/*
* we were idle so long that we count it as a perfect
* prediction
*/
new_factor += RESOLUTION;
/* Predict time until next break event */
data->current_predicted_us = max(measured_us, data->last_measured_us);
/*
* We don't want 0 as factor; we always want at least
* a tiny bit of estimated time.
*/
if (new_factor == 0)
new_factor = 1;
if (last_idle_us + BREAK_FUZZ <
data->expected_us - target->exit_latency) {
data->last_measured_us = measured_us;
data->elapsed_us = 0;
} else {
data->elapsed_us = measured_us;
}
data->correction_factor[data->bucket] = new_factor;
}
/**
......
......@@ -140,6 +140,10 @@ extern int nr_processes(void);
extern unsigned long nr_running(void);
extern unsigned long nr_uninterruptible(void);
extern unsigned long nr_iowait(void);
extern unsigned long nr_iowait_cpu(void);
extern unsigned long this_cpu_load(void);
extern void calc_global_load(void);
extern u64 cpu_nr_migrations(int cpu);
......
......@@ -2904,6 +2904,19 @@ unsigned long nr_iowait(void)
return sum;
}
unsigned long nr_iowait_cpu(void)
{
struct rq *this = this_rq();
return atomic_read(&this->nr_iowait);
}
unsigned long this_cpu_load(void)
{
struct rq *this = this_rq();
return this->cpu_load[0];
}
/* Variables and functions for calc_load */
static atomic_long_t calc_load_tasks;
static unsigned long calc_load_update;
......
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