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Jetpack/kernel_avc/kernel-4.9/rt-patches/0161-timers-Don-t-wake-ktim...

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From 71f9b17d0b1121b8219bd2752056ca1b00fa7db1 Mon Sep 17 00:00:00 2001
From: Haris Okanovic <haris.okanovic@ni.com>
Date: Fri, 3 Feb 2017 17:26:44 +0100
Subject: [PATCH 161/352] timers: Don't wake ktimersoftd on every tick
We recently upgraded from 4.1 to 4.6 and noticed a minor latency
regression caused by an additional thread wakeup (ktimersoftd) in
interrupt context on every tick. The wakeups are from
run_local_timers() raising TIMER_SOFTIRQ. Both TIMER and SCHED softirq
coalesced into one ksoftirqd wakeup prior to Sebastian's change to split
timers into their own thread.
There's already logic in run_local_timers() to avoid some unnecessary
wakeups of ksoftirqd, but it doesn't seems to catch them all. In
particular, I've seen many unnecessary wakeups when jiffies increments
prior to run_local_timers().
Change the way timers are collected per Julia and Thomas'
recommendation: Expired timers are now collected in interrupt context
and fired in ktimersoftd to avoid double-walk of `pending_map`.
Collect expired timers in interrupt context to avoid overhead of waking
ktimersoftd on every tick. ktimersoftd now wakes only when one or more
timers are ready, which yields a minor reduction in small latency spikes.
This is implemented by storing lists of expired timers in timer_base,
updated on each tick. Any addition to the lists wakes ktimersoftd
(softirq) to process those timers.
Signed-off-by: Haris Okanovic <haris.okanovic@ni.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/time/timer.c | 96 +++++++++++++++++++++++++++++++++++++----------------
1 file changed, 67 insertions(+), 29 deletions(-)
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index a416cee..c1b88e0 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -207,6 +207,8 @@ struct timer_base {
bool must_forward_clk;
DECLARE_BITMAP(pending_map, WHEEL_SIZE);
struct hlist_head vectors[WHEEL_SIZE];
+ struct hlist_head expired_lists[LVL_DEPTH];
+ int expired_count;
} ____cacheline_aligned;
static DEFINE_PER_CPU(struct timer_base, timer_bases[NR_BASES]);
@@ -1364,7 +1366,8 @@ static void call_timer_fn(struct timer_list *timer, void (*fn)(unsigned long),
}
}
-static void expire_timers(struct timer_base *base, struct hlist_head *head)
+static inline void __expire_timers(struct timer_base *base,
+ struct hlist_head *head)
{
while (!hlist_empty(head)) {
struct timer_list *timer;
@@ -1395,21 +1398,38 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
}
}
-static int __collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
+static void expire_timers(struct timer_base *base)
+{
+ struct hlist_head *head;
+
+ while (base->expired_count--) {
+ head = base->expired_lists + base->expired_count;
+ __expire_timers(base, head);
+ }
+ base->expired_count = 0;
+}
+
+static void __collect_expired_timers(struct timer_base *base)
{
unsigned long clk = base->clk;
struct hlist_head *vec;
- int i, levels = 0;
+ int i;
unsigned int idx;
+ /*
+ * expire_timers() must be called at least once before we can
+ * collect more timers
+ */
+ if (WARN_ON(base->expired_count))
+ return;
+
for (i = 0; i < LVL_DEPTH; i++) {
idx = (clk & LVL_MASK) + i * LVL_SIZE;
if (__test_and_clear_bit(idx, base->pending_map)) {
vec = base->vectors + idx;
- hlist_move_list(vec, heads++);
- levels++;
+ hlist_move_list(vec,
+ &base->expired_lists[base->expired_count++]);
}
/* Is it time to look at the next level? */
if (clk & LVL_CLK_MASK)
@@ -1417,7 +1437,6 @@ static int __collect_expired_timers(struct timer_base *base,
/* Shift clock for the next level granularity */
clk >>= LVL_CLK_SHIFT;
}
- return levels;
}
#ifdef CONFIG_NO_HZ_COMMON
@@ -1616,8 +1635,7 @@ void timer_clear_idle(void)
base->is_idle = false;
}
-static int collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
+static void collect_expired_timers(struct timer_base *base)
{
/*
* NOHZ optimization. After a long idle sleep we need to forward the
@@ -1634,20 +1652,49 @@ static int collect_expired_timers(struct timer_base *base,
if (time_after(next, jiffies)) {
/* The call site will increment clock! */
base->clk = jiffies - 1;
- return 0;
+ return;
}
base->clk = next;
}
- return __collect_expired_timers(base, heads);
+ __collect_expired_timers(base);
}
#else
-static inline int collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
+static inline void collect_expired_timers(struct timer_base *base)
{
- return __collect_expired_timers(base, heads);
+ __collect_expired_timers(base);
}
#endif
+static int find_expired_timers(struct timer_base *base)
+{
+ const unsigned long int end_clk = jiffies;
+
+ while (!base->expired_count && time_after_eq(end_clk, base->clk)) {
+ collect_expired_timers(base);
+ base->clk++;
+ }
+
+ return base->expired_count;
+}
+
+/* Called from CPU tick routine to quickly collect expired timers */
+static int tick_find_expired(struct timer_base *base)
+{
+ int count;
+
+ raw_spin_lock(&base->lock);
+
+ if (unlikely(time_after(jiffies, base->clk + HZ))) {
+ /* defer to ktimersoftd; don't spend too long in irq context */
+ count = -1;
+ } else
+ count = find_expired_timers(base);
+
+ raw_spin_unlock(&base->lock);
+
+ return count;
+}
+
/*
* Called from the timer interrupt handler to charge one tick to the current
* process. user_tick is 1 if the tick is user time, 0 for system.
@@ -1674,12 +1721,6 @@ void update_process_times(int user_tick)
*/
static inline void __run_timers(struct timer_base *base)
{
- struct hlist_head heads[LVL_DEPTH];
- int levels;
-
- if (!time_after_eq(jiffies, base->clk))
- return;
-
raw_spin_lock_irq(&base->lock);
/*
@@ -1698,14 +1739,9 @@ static inline void __run_timers(struct timer_base *base)
*/
base->must_forward_clk = false;
- while (time_after_eq(jiffies, base->clk)) {
-
- levels = collect_expired_timers(base, heads);
- base->clk++;
+ while (find_expired_timers(base))
+ expire_timers(base);
- while (levels--)
- expire_timers(base, heads + levels);
- }
raw_spin_unlock_irq(&base->lock);
wakeup_timer_waiters(base);
}
@@ -1731,12 +1767,12 @@ void run_local_timers(void)
hrtimer_run_queues();
/* Raise the softirq only if required. */
- if (time_before(jiffies, base->clk)) {
+ if (time_before(jiffies, base->clk) || !tick_find_expired(base)) {
if (!IS_ENABLED(CONFIG_NO_HZ_COMMON))
return;
/* CPU is awake, so check the deferrable base. */
base++;
- if (time_before(jiffies, base->clk))
+ if (time_before(jiffies, base->clk) || !tick_find_expired(base))
return;
}
raise_softirq(TIMER_SOFTIRQ);
@@ -1927,6 +1963,7 @@ int timers_dead_cpu(unsigned int cpu)
forward_timer_base(new_base);
BUG_ON(old_base->running_timer);
+ BUG_ON(old_base->expired_count);
for (i = 0; i < WHEEL_SIZE; i++)
migrate_timer_list(new_base, old_base->vectors + i);
@@ -1953,6 +1990,7 @@ static void __init init_timer_cpu(int cpu)
#ifdef CONFIG_PREEMPT_RT_FULL
init_swait_queue_head(&base->wait_for_running_timer);
#endif
+ base->expired_count = 0;
}
}
--
2.7.4
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