如何理解什么 extended quiescent state

也许先理解了 eqs 才可以理解 context_tracking 吧 ?

/*
 * Is the current CPU in an extended quiescent state?
 *
 * No ordering, as we are sampling CPU-local information.
 */
static __always_inline bool rcu_dynticks_curr_cpu_in_eqs(void)
{
	return !(raw_atomic_read(this_cpu_ptr(&context_tracking.state)) & RCU_DYNTICKS_IDX);
}

https://www.suse.com/c/cpu-isolation-full-dynticks-part2/

其中的 3.2 RCU quiescent states reporting 解释的很好:

1. 如果有 tick ，就没有必要使用这个概念，因为 : Because that would inflict a costly atomic operation with a full memory barrier on every user/kernel round trip. Also the duty to report quiescent states eventually falls to other CPUs so one must be aware of the exported cost that implies.
2. 如果没有，那么利用进入到 idle 和 userspace 可以知道，CPU 一定不是在 rcu critical region 中的

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