D.2.5 Towards a Greener RCU Implementation
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D.2.5 Towards a Greener RCU Implementation

As noted earlier, an important goal of this effort is to leave sleeping CPUs lie in order to promote energy conservation. In contrast, classic RCU will happily awaken each and every sleeping CPU at least once per grace period in some cases, which is suboptimal in the case where a small number of CPUs are busy doing RCU updates and the majority of the CPUs are mostly idle. This situation occurs frequently in systems sized for peak loads, and we need to be able to accommodate it gracefully. Furthermore, we need to fix a long-standing bug in Classic RCU where a dynticks-idle CPU servicing an interrupt containing a long-running RCU read-side critical section will fail to prevent an RCU grace period from ending.

Quick Quiz D.9: Given such an egregious bug, why does Linux run at all? End Quick Quiz

Figure: Hierarchical RCU State With Dynticks

This is accomplished by requiring that all CPUs manipulate counters located in a per-CPU rcu_dynticks structure. Loosely speaking, these counters have even-numbered values when the corresponding CPU is in dynticks idle mode, and have odd-numbered values otherwise. RCU thus needs to wait for quiescent states only for those CPUs whose rcu_dynticks counters are odd, and need not wake up sleeping CPUs, whose counters will be even. As shown in Figure , each per-CPU rcu_dynticks structure is shared by the ``rcu'' and ``rcu_bh'' implementations.

The following section presents a high-level view of the RCU state machine.