Understanding the linux kernel-ch6-Timing Measurements

 

6.2 The Linux Timekeeping Architecture
	· The kernel uses two basic timekeeping functions:
	one to keep the current time up-to-date and
	another to count the number of nanoseconds that have elapsed within the current second
	Data Structures of the Timekeeping Architecture
		· include/asm-i386/timer.h
		struct timer_opts {
			char* name;
			void (*mark_offset)(void);
			unsigned long (*get_offset)(void);
			unsigned long long (*monotonic_clock)(void);
			void (*delay)(unsigned long);
		};
		struct init_timer_opts {
			int (*init)(char *override);
			struct timer_opts *opts;
		};
		/* list of timers, ordered by preference, NULL terminated */
		static struct init_timer_opts* __initdata timers[] = {
		#ifdef CONFIG_X86_CYCLONE_TIMER
			&timer_cyclone_init,
		#endif
		#ifdef CONFIG_HPET_TIMER
			&timer_hpet_init,
		#endif
		#ifdef CONFIG_X86_PM_TIMER
			&timer_pmtmr_init,
		#endif
			&timer_tsc_init,
			&timer_pit_init,
			NULL,
		};
		/* iterates through the list of timers, returning the first
		* one that initializes successfully.
		*/
		struct timer_opts* __init select_timer(void)
		{
			int i = 0;
			/* find most preferred working timer */
			while (timers[i]) {
				if (timers[i]->init)
					if (timers[i]->init(clock_override) == 0)
						return timers[i]->opts;
				++i;
			}
			panic("select_timer: Cannot find a suitable timer/n");
			return NULL;
		}
		u64 get_jiffies_64(void)
		{
			unsigned long seq;
			u64 ret;
			do {
				seq = read_seqbegin(&xtime_lock);
				ret = jiffies_64;
			} while (read_seqretry(&xtime_lock, seq));
			return ret;
		}
		struct timespec {
			time_t	tv_sec;		/* seconds */
			long	tv_nsec;	/* nanoseconds */
		};
	·  Timekeeping Architecture in Uniprocessor Systems
		· Initialization phase
		void __init time_init(void)
		{
		#ifdef CONFIG_HPET_TIMER
			if (is_hpet_capable()) {
				/*
				* HPET initialization needs to do memory-mapped io. So, let
				* us do a late initialization after mem_init().
				*/
				late_time_init = hpet_time_init;
				return;
			}
		#endif
			xtime.tv_sec = get_cmos_time();
			xtime.tv_nsec = (INITIAL_JIFFIES % HZ) * (NSEC_PER_SEC / HZ);
			set_normalized_timespec(&wall_to_monotonic,
				-xtime.tv_sec, -xtime.tv_nsec);
			cur_timer = select_timer();
			printk(KERN_INFO "Using %s for high-res timesource/n",cur_timer->name);
			time_init_hook();
		}
		1. Initializes the xtime variable
		2. Initializes the wall_to_monotonic variable
		3. If the kernel supports HPET, it invokes the hpet_enable( ) function to determine
		whether the ACPI firmware has probed the chip and mapped
		its registers in the memory address space
		※hpet_enable( ) programs the first timer of the HPET chip so that it raises the
		IRQ 0 interrupt 1000 times per second. Otherwise, if the HPET chip is not available,
		the kernel will use the PIT: the chip has already been programmed by the init_IRQ( )
		function to raise 1000 timer interrupts per second
		4. Invokes select_timer( ) to select the best timer source available in the system,
		and sets the cur_timer variable to the address of the corresponding timer object
		5. Invokes setup_irq( 0,&irq0) to set up the interrupt gate corresponding to IRQ0
		the line associated with the system timer interrupt source (PIT or HPET)
		static struct irqaction irq0  = { timer_interrupt, SA_INTERRUPT, CPU_MASK_NONE, "timer", NULL, NULL};
		· The timer interrupt handler
		The timer_interrupt( ) function is the interrupt service routine (ISR)
		of the PIT or of the HPET; it performs the following steps:
		1. Protects the time-related kernel variables
		by issuing a write_seqlock() on the xtime_lock seqlock
		2. Executes the mark_offset method of the cur_timer timer object
		3. Invokes the do_timer_interrupt( ) function
		which in turn performs the following actions:
		a. Increases by one the value of jiffies_64
		b. Invokes the update_times( ) function to update
		the system date and time and to compute the current system load
		c. Invokes the update_process_times( ) function to perform several
		time-related accounting operations for the local CPU
		d. Invokes the profile_tick( ) function
		e. If the system clock is synchronized with an external clock
		invokes the set_rtc_mmss( ) function once every 660 seconds
		4. Releases the xtime_lock seqlock by invoking write_sequnlock().
		5. Returns the value 1 to notify that the interrupt has been effectively handled
	Timekeeping Architecture in Multiprocessor Systems
		Passed