+Although small-memory non-realtime systems can simply use Tiny RCU, +code size is only one aspect of memory efficiency. +Another aspect is the size of the rcu_head structure +used by call_rcu() and kfree_rcu(). +Although this structure contains nothing more than a pair of pointers, +it does appear in many RCU-protected data structures, including +some that are size critical. +The page structure is a case in point, as evidenced by +the many occurrences of the union keyword within that structure. + +
+This need for memory efficiency is one reason that RCU uses hand-crafted +singly linked lists to track the rcu_head structures that +are waiting for a grace period to elapse. +It is also the reason why rcu_head structures do not contain +debug information, such as fields tracking the file and line of the +call_rcu() or kfree_rcu() that posted them. +Although this information might appear in debug-only kernel builds at some +point, in the meantime, the ->func field will often provide +the needed debug information. + +
+However, in some cases, the need for memory efficiency leads to even +more extreme measures. +Returning to the page structure, the rcu_head field +shares storage with a great many other structures that are used at +various points in the corresponding page's lifetime. +In order to correctly resolve certain +race conditions, +the Linux kernel's memory-management subsystem needs a particular bit +to remain zero during all phases of grace-period processing, +and that bit happens to map to the bottom bit of the +rcu_head structure's ->next field. +RCU makes this guarantee as long as call_rcu() +is used to post the callback, as opposed to kfree_rcu() +or some future “lazy” +variant of call_rcu() that might one day be created for +energy-efficiency purposes. +
+Although small-memory non-realtime systems can simply use Tiny RCU, +code size is only one aspect of memory efficiency. +Another aspect is the size of the rcu_head structure +used by call_rcu() and kfree_rcu(). +Although this structure contains nothing more than a pair of pointers, +it does appear in many RCU-protected data structures, including +some that are size critical. +The page structure is a case in point, as evidenced by +the many occurrences of the union keyword within that structure. + +
+This need for memory efficiency is one reason that RCU uses hand-crafted +singly linked lists to track the rcu_head structures that +are waiting for a grace period to elapse. +It is also the reason why rcu_head structures do not contain +debug information, such as fields tracking the file and line of the +call_rcu() or kfree_rcu() that posted them. +Although this information might appear in debug-only kernel builds at some +point, in the meantime, the ->func field will often provide +the needed debug information. + +
+However, in some cases, the need for memory efficiency leads to even +more extreme measures. +Returning to the page structure, the rcu_head field +shares storage with a great many other structures that are used at +various points in the corresponding page's lifetime. +In order to correctly resolve certain +race conditions, +the Linux kernel's memory-management subsystem needs a particular bit +to remain zero during all phases of grace-period processing, +and that bit happens to map to the bottom bit of the +rcu_head structure's ->next field. +RCU makes this guarantee as long as call_rcu() +is used to post the callback, as opposed to kfree_rcu() +or some future “lazy” +variant of call_rcu() that might one day be created for +energy-efficiency purposes. +