On Mon, Nov 16 2020 at 21:24, Yunfeng Ye wrote: > On 2020/11/16 19:29, Thomas Gleixner wrote: >> There are quite some other inefficiencies in that code and the seqcount >> held time can be reduced way further. Let me stare at it. >> > I think the write seqcount only protecting the last_jiffies_update/jiffies_64/ > tick_next_period is enough. The modification which has not been tested, > look like this:
Yes, but it can be further simplified. I was doing that before I got distracted. Uncompiled and untested. A split up patch series is here: https://tglx.de/~tglx/patches-jiffies.tar Thanks, tglx --- --- a/include/linux/timer.h +++ b/include/linux/timer.h @@ -193,7 +193,6 @@ extern int try_to_del_timer_sync(struct #define del_singleshot_timer_sync(t) del_timer_sync(t) extern void init_timers(void); -extern void run_local_timers(void); struct hrtimer; extern enum hrtimer_restart it_real_fn(struct hrtimer *); --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -1693,29 +1693,6 @@ void timer_clear_idle(void) } #endif -/* - * 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. - */ -void update_process_times(int user_tick) -{ - struct task_struct *p = current; - - PRANDOM_ADD_NOISE(jiffies, user_tick, p, 0); - - /* Note: this timer irq context must be accounted for as well. */ - account_process_tick(p, user_tick); - run_local_timers(); - rcu_sched_clock_irq(user_tick); -#ifdef CONFIG_IRQ_WORK - if (in_irq()) - irq_work_tick(); -#endif - scheduler_tick(); - if (IS_ENABLED(CONFIG_POSIX_TIMERS)) - run_posix_cpu_timers(); -} - /** * __run_timers - run all expired timers (if any) on this CPU. * @base: the timer vector to be processed. @@ -1765,7 +1742,7 @@ static __latent_entropy void run_timer_s /* * Called by the local, per-CPU timer interrupt on SMP. */ -void run_local_timers(void) +static void run_local_timers(void) { struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]); @@ -1783,6 +1760,29 @@ void run_local_timers(void) } /* + * 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. + */ +void update_process_times(int user_tick) +{ + struct task_struct *p = current; + + PRANDOM_ADD_NOISE(jiffies, user_tick, p, 0); + + /* Note: this timer irq context must be accounted for as well. */ + account_process_tick(p, user_tick); + run_local_timers(); + rcu_sched_clock_irq(user_tick); +#ifdef CONFIG_IRQ_WORK + if (in_irq()) + irq_work_tick(); +#endif + scheduler_tick(); + if (IS_ENABLED(CONFIG_POSIX_TIMERS)) + run_posix_cpu_timers(); +} + +/* * Since schedule_timeout()'s timer is defined on the stack, it must store * the target task on the stack as well. */ --- a/kernel/time/tick-broadcast.c +++ b/kernel/time/tick-broadcast.c @@ -877,6 +877,21 @@ static void tick_broadcast_init_next_eve } } +static inline ktime_t tick_get_next_period(void) +{ + ktime_t next; + + /* + * Protect against concurrent updates (store /load tearing on + * 32bit). It does not matter if the time is already in the + * past. The broadcast device which is about to be programmed will + * fire in any case. + */ + raw_spin_lock(&jiffies_lock); + next = tick_next_period; + raw_spin_unlock(&jiffies_lock); +} + /** * tick_broadcast_setup_oneshot - setup the broadcast device */ @@ -905,10 +920,11 @@ static void tick_broadcast_setup_oneshot tick_broadcast_oneshot_mask, tmpmask); if (was_periodic && !cpumask_empty(tmpmask)) { + u64 ktime_t = tick_get_next_period(); + clockevents_switch_state(bc, CLOCK_EVT_STATE_ONESHOT); - tick_broadcast_init_next_event(tmpmask, - tick_next_period); - tick_broadcast_set_event(bc, cpu, tick_next_period); + tick_broadcast_init_next_event(tmpmask, nextevt); + tick_broadcast_set_event(bc, cpu, nextevt); } else bc->next_event = KTIME_MAX; } else { --- a/kernel/time/tick-common.c +++ b/kernel/time/tick-common.c @@ -27,7 +27,9 @@ */ DEFINE_PER_CPU(struct tick_device, tick_cpu_device); /* - * Tick next event: keeps track of the tick time + * Tick next event: keeps track of the tick time. It's updated by the + * CPU which handles the tick and protected by jiffies_lock. There is + * no requirement to write hold the jiffies seqcount for it. */ ktime_t tick_next_period; ktime_t tick_period; --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -44,7 +44,9 @@ struct tick_sched *tick_get_tick_sched(i #if defined(CONFIG_NO_HZ_COMMON) || defined(CONFIG_HIGH_RES_TIMERS) /* - * The time, when the last jiffy update happened. Protected by jiffies_lock. + * The time, when the last jiffy update happened. Write access must hold + * jiffies_lock and jiffies_seq. tick_nohz_next_event() needs to get a + * consistent view of jiffies and last_jiffies_update. */ static ktime_t last_jiffies_update; @@ -53,50 +55,59 @@ static ktime_t last_jiffies_update; */ static void tick_do_update_jiffies64(ktime_t now) { - unsigned long ticks = 0; + unsigned long ticks = 1; ktime_t delta; /* - * Do a quick check without holding jiffies_lock: - * The READ_ONCE() pairs with two updates done later in this function. + * Do a quick check without holding jiffies_lock. The READ_ONCE() + * pairs with the update done later in this function. */ - delta = ktime_sub(now, READ_ONCE(last_jiffies_update)); - if (delta < tick_period) + if (ktime_before(now, READ_ONCE(tick_next_period))) return; /* Reevaluate with jiffies_lock held */ raw_spin_lock(&jiffies_lock); + if (ktime_before(now, tick_next_period)) { + raw_spin_unlock(&jiffies_lock); + return; + } + write_seqcount_begin(&jiffies_seq); - delta = ktime_sub(now, last_jiffies_update); - if (delta >= tick_period) { + delta = ktime_sub(now, tick_next_period); + if (unlikely(delta >= tick_period)) { + /* Slow path for long idle sleep times */ + s64 incr = ktime_to_ns(tick_period); - delta = ktime_sub(delta, tick_period); - /* Pairs with the lockless read in this function. */ - WRITE_ONCE(last_jiffies_update, - ktime_add(last_jiffies_update, tick_period)); - - /* Slow path for long timeouts */ - if (unlikely(delta >= tick_period)) { - s64 incr = ktime_to_ns(tick_period); - - ticks = ktime_divns(delta, incr); - - /* Pairs with the lockless read in this function. */ - WRITE_ONCE(last_jiffies_update, - ktime_add_ns(last_jiffies_update, - incr * ticks)); - } - do_timer(++ticks); + ticks += ktime_divns(delta, incr); - /* Keep the tick_next_period variable up to date */ - tick_next_period = ktime_add(last_jiffies_update, tick_period); + last_jiffies_update = ktime_add_ns(last_jiffies_update, + incr * ticks); } else { - write_seqcount_end(&jiffies_seq); - raw_spin_unlock(&jiffies_lock); - return; + last_jiffies_update = ktime_add(last_jiffies_update, + tick_period); } + + /* + * Keep the tick_next_period variable up to date. WRITE_ONCE() + * pairs with the READ_ONCE() in the lockless quick check above. Do + * this here to make the lockless quick check above more efficient. + */ + WRITE_ONCE(tick_next_period, + ktime_add(last_jiffies_update, tick_period)); + + /* Advance jiffies to complete the jiffies_seq protected job */ + jiffies_64 += ticks; + + /* + * Release the sequence count. calc_global_load() below is not + * protected by it, but jiffies_lock needs to be held to prevent + * concurrent invocations. + */ write_seqcount_end(&jiffies_seq); + + calc_global_load(); + raw_spin_unlock(&jiffies_lock); update_wall_time(); }
