This is a counter implemented in every x86 microprocessor by means of a 64-bit register called TSC the register. It counts the number of clock signals arriving on the CLK pin of the processor. The current counter value can be read by accessing the TSC register. The number of ticks counted per second can be calculated as 1/(clock frequency); for a 1 GHz clock it translates to once every nanosecond.

Knowing the duration between two consecutive ticks is very crucial. The fact that one processor clock's frequency might not be the same as others makes it vary across processors. CPU clock frequency is calculated during system boot by the calibrate_tsc() callback routine of the x86_platform_ops structure defined in the arch/x86/include/asm/x86_init.h header file:

struct x86_platform_ops {
        unsigned long (*calibrate_cpu)(void);
        unsigned long (*calibrate_tsc)(void);
        void (*get_wallclock)(struct timespec *ts);
        int (*set_wallclock)(const struct timespec *ts);
        void (*iommu_shutdown)(void);
        bool (*is_untracked_pat_range)(u64 start, u64 end);
        void (*nmi_init)(void);
        unsigned char (*get_nmi_reason)(void);
        void (*save_sched_clock_state)(void);
        void (*restore_sched_clock_state)(void);
        void (*apic_post_init)(void);
        struct x86_legacy_features legacy;
        void (*set_legacy_features)(void);
};

This data structure manages other timing operations too, such as getting time from the RTC through get_wallclock() or setting time on the RTC through the set_wallclock() callback.