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Subject[RFC v2] timers: Don't wake ktimersoftd on every tick
Date
Changed 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`.

This is implemented by storing lists of expired timers in timer_base,
which carries a memory overhead 9*sizeof(pointer) per CPU. The timer
system uses hlist's which don't have end-node references, making it
impossible to merge 2 hlist's in constant time. I.e. Merging requires
walking one list. I also considered switching `vectors` to regular
list's which don't have this limitations, but that approach has the same
memory overhead. list_head is bigger than hlist_head by sizeof(pointer)
and is instantiated 9+ times per CPU as `vectors`. I believe the only
way to trim overhead is to spend more CPU cycles in interrupt context
either in list merging (unbounded operation) or the original double-walk
implementation. Any suggestions/preferences?

As before, a 6h run of cyclictest without CPU affinity shows decrease in
22-70us latency range. No change in max jitter. No change when `-S` is
used.

[Before/after traces]

ftp://ftp.ni.com/outgoing/tp02-timer-peek-traces.tgz
(Email me if link dies. Server periodically purges old files.)

[Hardware/software/config]

NI cRIO-9033
2 core Intel Atom CPU

Kernel 4.8.6-rt5
CONFIG_HZ_PERIODIC=y

[Outstanding concerns/issues/questions]

I'm relatively new to the timer subsystem, so please feel free to poke
as many holes as possible in this change. A few things that concern me
at the moment are:

Can jiffies change while one or more cpus is inside tick_sched_timer(),
in interrupt context? I'm copying jiffies to a local variable in
find_expired_timers() to ensure it doesn't run unbounded, but I'm not
sure if that's necessary.

Any special considerations for testing NO_HZ builds? (Other than letting
it run idle for a while)

timers_dead_cpu() presently asserts no timer callback is actively
running, which suggests that timers must be canceled prior to disabling
CPUs; otherwise, there's a race between active timers and hotplug
which can crash the whole kernel. Is this a safe assumption to make and
are there any special considerations for CPU hotplug testing?

Other tests/performance benchmark I should run?

Source: https://github.com/harisokanovic/linux/tree/dev/hokanovi/timer-peek-v2

Thanks,
Haris
---
kernel/time/timer.c | 97 ++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 67 insertions(+), 30 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index ba53447..b0cee45 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -206,6 +206,8 @@ struct timer_base {
bool is_idle;
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]);
@@ -1343,7 +1345,8 @@ 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;
@@ -1360,7 +1363,7 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
data = timer->data;

if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL) &&
- timer->flags & TIMER_IRQSAFE) {
+ timer->flags & TIMER_IRQSAFE) {
raw_spin_unlock(&base->lock);
call_timer_fn(timer, fn, data);
base->running_timer = NULL;
@@ -1374,21 +1377,37 @@ 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)
@@ -1396,7 +1415,6 @@ 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
@@ -1585,8 +1603,7 @@ 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
@@ -1603,20 +1620,49 @@ 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.
@@ -1643,22 +1689,11 @@ 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);

- 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);
}
@@ -1686,12 +1721,12 @@ 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) || !base->nohz_active)
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);
@@ -1861,6 +1896,7 @@ int timers_dead_cpu(unsigned int cpu)
raw_spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);

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);
@@ -1887,6 +1923,7 @@ 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;
}
}

--
2.10.1
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