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.okano...@ni.com> --- [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 https://github.com/harisokanovic/linux/tree/dev/hokanovi/timer-peek-v5 --- 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 5730d42bfd67..078027d8a866 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; @@ -209,6 +210,7 @@ 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]; } ____cacheline_aligned; static DEFINE_PER_CPU(struct timer_base, timer_bases[NR_BASES]); @@ -1314,7 +1316,8 @@ 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; @@ -1344,21 +1347,49 @@ 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) @@ -1366,7 +1397,8 @@ 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 @@ -1559,8 +1591,7 @@ 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 @@ -1581,17 +1612,48 @@ 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. */ @@ -1618,22 +1680,12 @@ 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); } @@ -1661,12 +1713,12 @@ 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); @@ -1826,6 +1878,7 @@ 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); -- 2.13.2
