sched_clock

参考 https://www.kernel.org/doc/html/latest/timers/timekeeping.html

As the name suggests, sched_clock() is used for scheduling the system, determining the absolute timeslice for a certain process in the CFS scheduler for example. It is also used for printk timestamps when you have selected to include time information in printk for things like bootcharts.

notrace u64 sched_clock(void)
{
	u64 now;
	preempt_disable_notrace();
	now = sched_clock_noinstr();
	preempt_enable_notrace();
	return now;
}

sched_clock_noinstr 直接调用到 kvm_sched_clock_read 或者 native_sched_clock ，其实就是这 rdstsc 而已。

关于 cpu_clock 的问题

/*
 * Similar to cpu_clock(), but requires local IRQs to be disabled.
 *
 * See cpu_clock().
 */
notrace u64 sched_clock_cpu(int cpu)

static inline u64 cpu_clock(int cpu)
{
	return sched_clock_cpu(cpu);
}

注释看， cpu_clock 和 sched_clock_cpu 没有任何区别啊?

为什么一个需要 local IRQs disable ，一个不可以

sched_clock_cpu 其实没有考虑 CPU 参数啊

[    0.622130] sched_clock: Marking stable (576000952, 45923638)->(655351796, -33427206)

sched_clock_cpu 中:

	if (sched_clock_stable())
		return sched_clock() + __sched_clock_offset;

或者说，当一个程序切换到另外一个 CPU 之后，那么时间就开始切换一下。

sysvec_call_function_single
- instr_sysvec_call_function_single
  - __sysvec_call_function_single
    - generic_smp_call_function_single_interrupt
      - __flush_smp_call_function_queue
        
        csd_do_func
        
        sched_ttwu_pending
        
        ttwu_do_activate
        
        activate_task
        
        enqueue_task
        
        psi_enqueue
        
        psi_task_change
        
        psi_group_change
        
        cpu_clock

感觉就是介绍了半天，实际上啥也没有。

TODO

windows 果然是有自己的想法的

hv_setup_sched_clock

附录: sched/clock.c 的注释

/*
 * sched_clock() for unstable CPU clocks

 * What this file implements:
 *
 * cpu_clock(i) provides a fast (execution time) high resolution
 * clock with bounded drift between CPUs. The value of cpu_clock(i)
 * is monotonic for constant i. The timestamp returned is in nanoseconds.
 *
 * ######################### BIG FAT WARNING ##########################
 * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
 * # go backwards !!                                                  #
 * ####################################################################
 *
 * There is no strict promise about the base, although it tends to start
 * at 0 on boot (but people really shouldn't rely on that).
 *
 * cpu_clock(i)       -- can be used from any context, including NMI.
 * local_clock()      -- is cpu_clock() on the current CPU.
 *
 * sched_clock_cpu(i)
 *
 * How it is implemented:
 *
 * The implementation either uses sched_clock() when
 * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
 * sched_clock() is assumed to provide these properties (mostly it means
 * the architecture provides a globally synchronized highres time source).
 *
 * Otherwise it tries to create a semi stable clock from a mixture of other
 * clocks, including:
 *
 *  - GTOD (clock monotonic)
 *  - sched_clock()
 *  - explicit idle events
 *
 * We use GTOD as base and use sched_clock() deltas to improve resolution. The
 * deltas are filtered to provide monotonicity and keeping it within an
 * expected window.
 *
 * Furthermore, explicit sleep and wakeup hooks allow us to account for time
 * that is otherwise invisible (TSC gets stopped).
 *
 */

cpu_clock 用于封装 unstable cpu clocks 的

cpu_clock(i) provides a fast (execution time) high resolution clock with bounded drift between CPUs. The value of cpu_clock(i)

等待整理

通用 sched clock 模块。这个模块主要是提供一个 sched_clock 的接口函数，调用该函数可以获取当前时间点到系统启动之间的纳秒值。底层的 HW counter 其实是千差万别的，有些平台可以提供 64-bit 的 HW counter，因此，在那样的平台中，我们可以不使用这个通用 sched clock 模块（不配置 CONFIG_GENERIC_SCHED_CLOCK 这个内核选项），而在自己的 clock source chip driver 中直接提供 sched_clock 接口。使用通用 sched clock 模块的好处是：该模块扩展了 64-bit 的 counter，即使底层的 HW counter 比特数目不足（有些平台 HW counter 只有 32 个 bit）。

CONFIG_GENERIC_SCHED_CLOCK is not set in x86 defconfig

这是为什么?

算加上 clocksource=tsc tsc=reliable，重启后， guest sched clock 还是 kvmclock (edited)

sched_clock 作用到底是什么?

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