lkml.org 
[lkml]   [2018]   [Mar]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[RFC/RFT][PATCH 6/7] sched: idle: Predict idle duration before stopping the tick
Date
From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

In order to address the issue with short idle duration predictions
by the idle governor after the tick has been stopped, reorder the
code in cpuidle_idle_call() so that the governor idle state selection
runs before tick_nohz_idle_go_idle() and use the expected idle period
duration returned by cpuidle_select() to tell tick_nohz_idle_go_idle()
whether or not to stop the tick.

This isn't straightforward, because menu_predict() invokes
tick_nohz_get_sleep_length() to get the time to the next timer
event and the number returned by the latter comes from
__tick_nohz_idle_enter(). Fortunately, however, it is possible
to compute that number without actually stopping the tick and with
the help of the existing code.

Namely, notice that tick_nohz_stop_sched_tick() already computes the
next timer event time to reprogram the scheduler tick hrtimer and
that time can be used as a proxy for the actual next timer event
time in the idle duration predicition.

Accordingly, rename the original tick_nohz_stop_sched_tick() to
__tick_nohz_next_event() and add the stop_tick argument indicating
whether or not to stop the tick to it. If that argument is 'true',
the function will work like the original tick_nohz_stop_sched_tick(),
but otherwise it will just compute the next event time without
stopping the tick. Next, redefine tick_nohz_stop_sched_tick() as
a wrapper around the new function.

Following that, make tick_nohz_get_sleep_length() call
__tick_nohz_next_event() to compute the next timer event time
and make it use the new last_jiffies_update field in struct
tick_sched to tell __tick_nohz_idle_enter() to skip some code
that has run already.

[After this change the __tick_nohz_next_event() code computing the
next event time will run twice in a row if the expected idle period
duration coming from cpuidle_select() is large enough which is sort
of ugly, but the next set of changes deals with that separately.
To do that, it uses the value of the last_jiffies_update field in
struct tick_sched introduced here, among other things.]

Finally, drop the now redundant sleep_length field from struct
tick_sched.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
kernel/sched/idle.c | 7 ++---
kernel/time/tick-sched.c | 64 +++++++++++++++++++++++++++++++++--------------
kernel/time/tick-sched.h | 3 --
3 files changed, 50 insertions(+), 24 deletions(-)

Index: linux-pm/kernel/sched/idle.c
===================================================================
--- linux-pm.orig/kernel/sched/idle.c
+++ linux-pm/kernel/sched/idle.c
@@ -188,13 +188,14 @@ static void cpuidle_idle_call(void)
} else {
unsigned int duration_us;

- tick_nohz_idle_go_idle(true);
- rcu_idle_enter();
-
/*
* Ask the cpuidle framework to choose a convenient idle state.
*/
next_state = cpuidle_select(drv, dev, &duration_us);
+
+ tick_nohz_idle_go_idle(duration_us > USEC_PER_SEC / HZ);
+ rcu_idle_enter();
+
entered_state = call_cpuidle(drv, dev, next_state);
/*
* Give the governor an opportunity to reflect on the outcome
Index: linux-pm/kernel/time/tick-sched.c
===================================================================
--- linux-pm.orig/kernel/time/tick-sched.c
+++ linux-pm/kernel/time/tick-sched.c
@@ -655,8 +655,8 @@ static inline bool local_timer_softirq_p
return local_softirq_pending() & TIMER_SOFTIRQ;
}

-static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
- ktime_t now, int cpu)
+static ktime_t __tick_nohz_next_event(struct tick_sched *ts, int cpu,
+ bool stop_tick)
{
struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
@@ -670,6 +670,7 @@ static ktime_t tick_nohz_stop_sched_tick
basejiff = jiffies;
} while (read_seqretry(&jiffies_lock, seq));
ts->last_jiffies = basejiff;
+ ts->last_jiffies_update = basemono;

/*
* Keep the periodic tick, when RCU, architecture or irq_work
@@ -732,8 +733,10 @@ static ktime_t tick_nohz_stop_sched_tick
*/
delta = timekeeping_max_deferment();
if (cpu == tick_do_timer_cpu) {
- tick_do_timer_cpu = TICK_DO_TIMER_NONE;
- ts->do_timer_last = 1;
+ if (stop_tick) {
+ tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ ts->do_timer_last = 1;
+ }
} else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
delta = KTIME_MAX;
ts->do_timer_last = 0;
@@ -756,6 +759,12 @@ static ktime_t tick_nohz_stop_sched_tick
expires = min_t(u64, expires, next_tick);
tick = expires;

+ if (!stop_tick) {
+ /* Undo the effect of get_next_timer_interrupt(). */
+ timer_clear_idle();
+ goto out;
+ }
+
/* Skip reprogram of event if its not changed */
if (ts->tick_stopped && (expires == ts->next_tick)) {
/* Sanity check: make sure clockevent is actually programmed */
@@ -804,14 +813,14 @@ static ktime_t tick_nohz_stop_sched_tick
else
tick_program_event(tick, 1);
out:
- /*
- * Update the estimated sleep length until the next timer
- * (not only the tick).
- */
- ts->sleep_length = ktime_sub(dev->next_event, now);
return tick;
}

+static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts, int cpu)
+{
+ return __tick_nohz_next_event(ts, cpu, true);
+}
+
static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
{
/* Update jiffies first */
@@ -847,7 +856,7 @@ static void tick_nohz_full_update_tick(s
return;

if (can_stop_full_tick(cpu, ts))
- tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
+ tick_nohz_stop_sched_tick(ts, cpu);
else if (ts->tick_stopped)
tick_nohz_restart_sched_tick(ts, ktime_get());
#endif
@@ -873,10 +882,8 @@ static bool can_stop_idle_tick(int cpu,
return false;
}

- if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) {
- ts->sleep_length = NSEC_PER_SEC / HZ;
+ if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
return false;
- }

if (need_resched())
return false;
@@ -913,17 +920,22 @@ static bool can_stop_idle_tick(int cpu,

static void __tick_nohz_idle_enter(struct tick_sched *ts, bool stop_tick)
{
- ktime_t now, expires;
int cpu = smp_processor_id();

- now = tick_nohz_start_idle(ts);
+ if (!ts->last_jiffies_update) {
+ /* tick_nohz_get_sleep_length() has not run. */
+ tick_nohz_start_idle(ts);
+ if (!can_stop_idle_tick(cpu, ts))
+ return;
+ }

- if (can_stop_idle_tick(cpu, ts) && stop_tick) {
+ if (stop_tick) {
int was_stopped = ts->tick_stopped;
+ ktime_t expires;

ts->idle_calls++;

- expires = tick_nohz_stop_sched_tick(ts, now, cpu);
+ expires = tick_nohz_stop_sched_tick(ts, cpu);
if (expires > 0LL) {
ts->idle_sleeps++;
ts->idle_expires = expires;
@@ -934,6 +946,8 @@ static void __tick_nohz_idle_enter(struc
nohz_balance_enter_idle(cpu);
}
}
+
+ ts->last_jiffies_update = 0;
}

void __tick_nohz_idle_prepare(void)
@@ -1006,15 +1020,27 @@ void tick_nohz_irq_exit(void)
}

/**
- * tick_nohz_get_sleep_length - return the length of the current sleep
+ * tick_nohz_get_sleep_length - return the expected length of the current sleep
*
* Called from power state control code with interrupts disabled
*/
ktime_t tick_nohz_get_sleep_length(void)
{
struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
+ int cpu = smp_processor_id();
+ ktime_t now, next_event;

- return ts->sleep_length;
+ now = tick_nohz_start_idle(ts);
+
+ if (can_stop_idle_tick(cpu, ts)) {
+ next_event = __tick_nohz_next_event(ts, cpu, false);
+ } else {
+ struct clock_event_device *dev;
+
+ dev = __this_cpu_read(tick_cpu_device.evtdev);
+ next_event = dev->next_event;
+ }
+ return ktime_sub(next_event, now);;
}

/**
Index: linux-pm/kernel/time/tick-sched.h
===================================================================
--- linux-pm.orig/kernel/time/tick-sched.h
+++ linux-pm/kernel/time/tick-sched.h
@@ -38,7 +38,6 @@ enum tick_nohz_mode {
* @idle_exittime: Time when the idle state was left
* @idle_sleeptime: Sum of the time slept in idle with sched tick stopped
* @iowait_sleeptime: Sum of the time slept in idle with sched tick stopped, with IO outstanding
- * @sleep_length: Duration of the current idle sleep
* @do_timer_lst: CPU was the last one doing do_timer before going idle
*/
struct tick_sched {
@@ -58,8 +57,8 @@ struct tick_sched {
ktime_t idle_exittime;
ktime_t idle_sleeptime;
ktime_t iowait_sleeptime;
- ktime_t sleep_length;
unsigned long last_jiffies;
+ u64 last_jiffies_update;
u64 next_timer;
ktime_t idle_expires;
int do_timer_last;
\
 
 \ /
  Last update: 2018-03-04 23:32    [W:0.122 / U:2.708 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site