Re: [PATCH] Revert "timers: Don't wake ktimersoftd on every tick"
From: Haris Okanovic
Date: Fri May 26 2017 - 15:28:45 EST
Anna-Maria,
Look-ahead is implemented by tick_find_expired() and expiry by
__run_timers(), both of which hold timer_base::lock (raw spin lock)
while running. Those two routines shouldn't be able to run
simultaneously on the same timer_base. Are you sure the race isn't in
another code path?
Do you have a test which demonstrates this? I'd like to investigate.
Your .config file would also be useful.
I'm looking at v4.9.27-rt17 source.
Thanks,
Haris
On 05/26/2017 12:16 PM, Anna-Maria Gleixner wrote:
This reverts commit 032f93cae150a.
The problem is that the look ahead optimization from the tick timer
interrupt context can race with the softirq thread expiring timer. As
a consequence the temporary hlist heads which hold the to expire
timers are overwritten and the timers which are already removed from
the wheel bucket for expiry are now dangling w/o a list head.
That means those timers never get expired. If one of those timers is
canceled the removal operation will result in a hlist corruption.
Signed-off-by: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx>
---
kernel/time/timer.c | 96 ++++++++++++++++-------------------------------------
1 file changed, 29 insertions(+), 67 deletions(-)
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index cdff4411f8f6..08a5ab762495 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -206,8 +206,6 @@ 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]);
@@ -1355,8 +1353,7 @@ static void call_timer_fn(struct timer_list *timer, void (*fn)(unsigned long),
}
}
-static inline 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;
@@ -1387,38 +1384,21 @@ static inline void __expire_timers(struct timer_base *base,
}
}
-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)
+static int __collect_expired_timers(struct timer_base *base,
+ struct hlist_head *heads)
{
unsigned long clk = base->clk;
struct hlist_head *vec;
- int i;
+ int i, levels = 0;
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,
- &base->expired_lists[base->expired_count++]);
+ hlist_move_list(vec, heads++);
+ levels++;
}
/* Is it time to look at the next level? */
if (clk & LVL_CLK_MASK)
@@ -1426,6 +1406,7 @@ static void __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
@@ -1618,7 +1599,8 @@ void timer_clear_idle(void)
base->is_idle = false;
}
-static void collect_expired_timers(struct timer_base *base)
+static int collect_expired_timers(struct timer_base *base,
+ struct hlist_head *heads)
{
/*
* NOHZ optimization. After a long idle sleep we need to forward the
@@ -1635,49 +1617,20 @@ static void collect_expired_timers(struct timer_base *base)
if (time_after(next, jiffies)) {
/* The call site will increment clock! */
base->clk = jiffies - 1;
- return;
+ return 0;
}
base->clk = next;
}
- __collect_expired_timers(base);
+ return __collect_expired_timers(base, heads);
}
#else
-static inline void collect_expired_timers(struct timer_base *base)
+static inline int collect_expired_timers(struct timer_base *base,
+ struct hlist_head *heads)
{
- __collect_expired_timers(base);
+ return __collect_expired_timers(base, heads);
}
#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.
@@ -1704,11 +1657,22 @@ 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 (find_expired_timers(base))
- expire_timers(base);
+ while (time_after_eq(jiffies, base->clk)) {
+
+ 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);
}
@@ -1736,12 +1700,12 @@ void run_local_timers(void)
hrtimer_run_queues();
/* Raise the softirq only if required. */
- if (time_before(jiffies, base->clk) || !tick_find_expired(base)) {
+ if (time_before(jiffies, base->clk)) {
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) || !tick_find_expired(base))
+ if (time_before(jiffies, base->clk))
return;
}
raise_softirq(TIMER_SOFTIRQ);
@@ -1911,7 +1875,6 @@ 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);
@@ -1938,7 +1901,6 @@ 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;
}
}