时间管理(四) ---- SMP更新tick职责处理

v3.14.25

系统初始化时会设定哪个CPU更新 jiffies,谁先运行了这段代码谁负责更新

166 static void tick_setup_device(struct tick_device *td,
167 +---  2 lines: struct clock_event_device *newdev, int cpu,--------
169 {170 +--  6 lines: ktime_t next_event;----------------------------------
176     if (!td->evtdev) {177 +--  4 lines: If no cpu took the do_timer update, assign it to------
181         if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
182             if (!tick_nohz_full_cpu(cpu))
183                 tick_do_timer_cpu = cpu;
184             else    
185                 tick_do_timer_cpu = TICK_DO_TIMER_NONE;
186             tick_next_period = ktime_get();
187             tick_period = ktime_set(0, NSEC_PER_SEC / HZ);
188         }       189 +-- 34 lines: Startup in periodic mode first.------------------------
223 }

负责更新 jiffies的CPU 在进入idle时会“放弃”更新 jiffies的职责

530 static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
531                      ktime_t now, int cpu)
532
{533 +-- 36 lines: unsigned long seq, last_jiffies, next_jiffies, delta_jiffies;---
569     /* Schedule the tick, if we are at least one jiffie off */
570     if ((long)delta_jiffies >= 1) {
571
572 +-- 13 lines: If this cpu is the one which updates jiffies, then--
585         if (cpu == tick_do_timer_cpu) {
586             tick_do_timer_cpu = TICK_DO_TIMER_NONE;
587             ts-do_timer_last = 1;
588         } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
589             time_delta = KTIME_MAX;
590             ts->do_timer_last = 0;591         } else if (!ts-do_timer_last) {
592             time_delta = KTIME_MAX;593         }
594 +-- 86 lines: #ifdef CONFIG_NO_HZ_FULL-----------------------------
680 }

其它CPU在处理 timer tick中断时会 检测是否有CPU负责更新jiffies,如果没有则自己负责更新jiffies.

112 static void tick_sched_do_timer(ktime_t now)
113 {114 +-- 10 lines: int cpu = smp_processor_id();-----------
124     if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)
125         && !tick_nohz_full_cpu(cpu))
126         tick_do_timer_cpu = cpu;

127 +--  3 lines: #endif---------------------------------------
130     if (tick_do_timer_cpu == cpu) 
131         tick_do_update_jiffies64(now);
132 }