[PATCH v4 1/2] timers: Don't wake ktimersoftd on every tick

From: Haris Okanovic
Date: Tue Mar 06 2018 - 18:40:12 EST


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 measure by cyclictest.

Execution time of run_local_timers() increases by 0.2us to 2.5us as
measured by TSC on a 2core Intel Atom E3825 system.

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@xxxxxx>
---
[PATCH v2] Applied Thomas Gleixner's suggestions:
- Fix expired_count race
- Remove unneeded base->clk lookahead
- Return expired_count in collect_expired_timers()
- Add block_softirq
- Rebase to v4.11.8-rt5
[PATCH v3]
- Fix cosmetic issues
- Rename "count" to "levels" in timer_base and various functions
- Move expired_levels and block_softirq to fill holes in timer_base
- Remove READ_ONCE/WRITE_ONCE around block_softirq
[PATCH v4]
- Rebase onto v4.14.20-rt17

https://github.com/harisokanovic/linux/tree/dev/hokanovi/timer-peek-v6
---
kernel/time/timer.c | 111 ++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 82 insertions(+), 29 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 96fd01c9f6b1..98e952a6428d 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -197,6 +197,7 @@ EXPORT_SYMBOL(jiffies_64);

struct timer_base {
raw_spinlock_t lock;
+ int expired_levels;
struct timer_list *running_timer;
#ifdef CONFIG_PREEMPT_RT_FULL
struct swait_queue_head wait_for_running_timer;
@@ -208,6 +209,7 @@ 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];
} ____cacheline_aligned;

static DEFINE_PER_CPU(struct timer_base, timer_bases[NR_BASES]);
@@ -1342,7 +1344,8 @@ 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 void __expire_timers(struct timer_base *base,
+ struct hlist_head *head)
{
while (!hlist_empty(head)) {
struct timer_list *timer;
@@ -1372,21 +1375,49 @@ 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;
+ int levels = base->expired_levels;
+
+ while (levels--) {
+ head = base->expired_lists + levels;
+ __expire_timers(base, head);
+ }
+
+ /*
+ * Zero base->expired_levels after processing all base->expired_lists
+ * to signal it's ready to get re-populated. Otherwise, we race with
+ * tick_find_expired() when base->lock is temporarily dropped in
+ * __expire_timers()
+ */
+ base->expired_levels = 0;
+}
+
+static int __collect_expired_timers(struct timer_base *base)
{
- unsigned long clk = base->clk;
struct hlist_head *vec;
- int i, levels = 0;
+ struct hlist_head *expired_list = base->expired_lists;
+ unsigned long clk;
+ int i;
unsigned int idx;

+ /*
+ * expire_timers() must be called at least once before we can
+ * collect more timers.
+ */
+ if (base->expired_levels)
+ return base->expired_levels;
+
+ clk = base->clk;
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, expired_list);
+ base->expired_levels++;
+ expired_list++;
}
/* Is it time to look at the next level? */
if (clk & LVL_CLK_MASK)
@@ -1394,7 +1425,8 @@ static int __collect_expired_timers(struct timer_base *base,
/* Shift clock for the next level granularity */
clk >>= LVL_CLK_SHIFT;
}
- return levels;
+
+ return base->expired_levels;
}

#ifdef CONFIG_NO_HZ_COMMON
@@ -1593,8 +1625,7 @@ void timer_clear_idle(void)
base->is_idle = false;
}

-static int collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
+static int collect_expired_timers(struct timer_base *base)
{
/*
* NOHZ optimization. After a long idle sleep we need to forward the
@@ -1615,16 +1646,47 @@ static int collect_expired_timers(struct timer_base *base,
}
base->clk = next;
}
- return __collect_expired_timers(base, heads);
+ return __collect_expired_timers(base);
}
#else
-static inline int collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
+static inline int collect_expired_timers(struct timer_base *base)
{
- return __collect_expired_timers(base, heads);
+ return __collect_expired_timers(base);
}
#endif

+/*
+ * Increments timer_base to current jiffies or until first expired
+ * timer is found. Return number of expired levels.
+ */
+static int find_expired_timers(struct timer_base *base)
+{
+ unsigned long int end_clk = jiffies;
+ int expired_levels;
+
+ while (!(expired_levels = collect_expired_timers(base)) &&
+ time_after_eq(end_clk, base->clk)) {
+ base->clk++;
+ }
+
+ return expired_levels;
+}
+
+/*
+ * Called from CPU tick routine to collect expired timers up to current
+ * jiffies. Return number of expired levels.
+ */
+static int tick_find_expired(struct timer_base *base)
+{
+ int levels;
+
+ raw_spin_lock(&base->lock);
+ levels = find_expired_timers(base);
+ raw_spin_unlock(&base->lock);
+
+ return levels;
+}
+
/*
* 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.
@@ -1652,22 +1714,12 @@ 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)) {
+ do {
+ expire_timers(base);
+ } while (find_expired_timers(base));

- levels = collect_expired_timers(base, heads);
- base->clk++;
-
- while (levels--)
- expire_timers(base, heads + levels);
- }
raw_spin_unlock_irq(&base->lock);
wakeup_timer_waiters(base);
}
@@ -1707,12 +1759,12 @@ 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);
@@ -1887,6 +1939,7 @@ 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_levels);

for (i = 0; i < WHEEL_SIZE; i++)
migrate_timer_list(new_base, old_base->vectors + i);
--
2.15.1