linux时间子系统 - clocksource/timekeeper

article/2025/9/12 11:59:45

linux时间子系统中有一个很重要的记录时间的模块就是timekeeper，而timekeeper需要底层clock硬件设备的支持，每一个设备用结构体clocksource来表示，注册进系统的每一个clocksource会凭借优先级最终被选择成为timekeeper的时钟源。

1. clocksource的注册

1.1 clocksource结构体

struct clocksource {cycle_t (*read)(struct clocksource *cs);cycle_t mask;u32 mult;u32 shift;u64 max_idle_ns;u32 maxadj;
#ifdef CONFIG_ARCH_CLOCKSOURCE_DATAstruct arch_clocksource_data archdata;
#endifu64 max_cycles;const char *name;struct list_head list;int rating;----------------------------------------clock设备的优先级int (*enable)(struct clocksource *cs);-------------clock设备使能void (*disable)(struct clocksource *cs);-----------clock设备关闭unsigned long flags;-------------------------------clock源的标志void (*suspend)(struct clocksource *cs);-----------PM调用函数suspendvoid (*resume)(struct clocksource *cs);------------PM调用函数resume/* private: */
#ifdef CONFIG_CLOCKSOURCE_WATCHDOG/* Watchdog related data, used by the framework */struct list_head wd_list;cycle_t cs_last;cycle_t wd_last;
#endifstruct module *owner;
};

1.2 clocksource_list

每一个注册进系统的clock设备都会加入一个全局链表clocksource_list

（kernel/time/clocksource.c）

static LIST_HEAD(clocksource_list);

但是timekeeper只会选取其中最合适的一个作为主clock，用一下变量来表示

static struct clocksource *curr_clocksource;

1.3 注册函数

clocksource的注册函数有如下几个：

（include/linux/clocksource.h）

static inline int __clocksource_register(struct clocksource *cs)
{return __clocksource_register_scale(cs, 1, 0);
}static inline int clocksource_register_hz(struct clocksource *cs, u32 hz)
{return __clocksource_register_scale(cs, 1, hz);
}static inline int clocksource_register_khz(struct clocksource *cs, u32 khz)
{

1.4 clocksource注册过程

clocksource设备的注册可以当作一个驱动，所以一般clock设备的文件都在dricers/clocksource/目录下，下面以arm global timer为例来介绍注册过程。

clocksource设备的初始化接口由宏定义：

CLOCKSOURCE_OF_DECLARE(arm_gt, "arm,cortex-a9-global-timer",global_timer_of_register);

global的初始化函数为global_timer_of_register，下图是注册的函数调用关系图：

这里写图片描述

1.4.1 把clock设备加入clocksource_list

函数clocksource_enqueue实现这个过程

static void clocksource_enqueue(struct clocksource *cs)
{struct list_head *entry = &clocksource_list;struct clocksource *tmp;list_for_each_entry(tmp, &clocksource_list, list)/* Keep track of the place, where to insert */if (tmp->rating >= cs->rating)entry = &tmp->list;--------------得到最合适插入的地方list_add(&cs->list, entry);--------------加入clocksource_list
}

1.4.2 选取最合适的curr_clocksource

函数__clocksource_select实现这个过程：

static void __clocksource_select(bool skipcur)
{bool oneshot = tick_oneshot_mode_active();struct clocksource *best, *cs;/* Find the best suitable clocksource */best = clocksource_find_best(oneshot, skipcur);if (!best)
        return;--------------------------------------------没有找到合适的就直接返回/* Check for the override clocksource. */list_for_each_entry(cs, &clocksource_list, list) {if (skipcur && cs == curr_clocksource)continue;if (strcmp(cs->name, override_name) != 0)continue;/** Check to make sure we don't switch to a non-highres* capable clocksource if the tick code is in oneshot* mode (highres or nohz)*/if (!(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) && oneshot) {/* Override clocksource cannot be used. */pr_warn("Override clocksource %s is not HRT compatible - cannot switch while in HRT/NOHZ mode\n",cs->name);override_name[0] = 0;} else/* Override clocksource can be used. */best = cs;-------------------------------------根据上面各个条件的过滤，最后得到最好的clocksourcebreak;}if (curr_clocksource != best && !timekeeping_notify(best)) {pr_info("Switched to clocksource %s\n", best->name);curr_clocksource = best;--------------------------把clocksource给curr_clocksource }
}

2. timekeeper

struct timekeeper {struct tk_read_base tkr_mono;struct tk_read_base tkr_raw;u64         xtime_sec;unsigned long       ktime_sec;struct timespec64   wall_to_monotonic;ktime_t         offs_real;ktime_t         offs_boot;ktime_t         offs_tai;s32         tai_offset;unsigned int        clock_was_set_seq;u8          cs_was_changed_seq;ktime_t         next_leap_ktime;struct timespec64   raw_time;/* The following members are for timekeeping internal use */cycle_t         cycle_interval;u64         xtime_interval;s64         xtime_remainder;u32         raw_interval;/* The ntp_tick_length() value currently being used.* This cached copy ensures we consistently apply the tick* length for an entire tick, as ntp_tick_length may change* mid-tick, and we don't want to apply that new value to* the tick in progress.*/u64         ntp_tick;/* Difference between accumulated time and NTP time in ntp* shifted nano seconds. */s64         ntp_error;u32         ntp_error_shift;u32         ntp_err_mult;
#ifdef CONFIG_DEBUG_TIMEKEEPINGlong            last_warning;/** These simple flag variables are managed* without locks, which is racy, but they are* ok since we don't really care about being* super precise about how many events were* seen, just that a problem was observed.*/int         underflow_seen;int         overflow_seen;
#endif
};