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Diffstat (limited to 'Documentation/RCU/checklist.txt')
| -rw-r--r-- | Documentation/RCU/checklist.txt | 44 |
1 files changed, 41 insertions, 3 deletions
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 49e27cc1938..1d50cf0c905 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -144,9 +144,47 @@ over a rather long period of time, but improvements are always welcome! whether the increased speed is worth it. 8. Although synchronize_rcu() is a bit slower than is call_rcu(), - it usually results in simpler code. So, unless update performance - is important or the updaters cannot block, synchronize_rcu() - should be used in preference to call_rcu(). + it usually results in simpler code. So, unless update + performance is critically important or the updaters cannot block, + synchronize_rcu() should be used in preference to call_rcu(). + + An especially important property of the synchronize_rcu() + primitive is that it automatically self-limits: if grace periods + are delayed for whatever reason, then the synchronize_rcu() + primitive will correspondingly delay updates. In contrast, + code using call_rcu() should explicitly limit update rate in + cases where grace periods are delayed, as failing to do so can + result in excessive realtime latencies or even OOM conditions. + + Ways of gaining this self-limiting property when using call_rcu() + include: + + a. Keeping a count of the number of data-structure elements + used by the RCU-protected data structure, including those + waiting for a grace period to elapse. Enforce a limit + on this number, stalling updates as needed to allow + previously deferred frees to complete. + + Alternatively, limit only the number awaiting deferred + free rather than the total number of elements. + + b. Limiting update rate. For example, if updates occur only + once per hour, then no explicit rate limiting is required, + unless your system is already badly broken. The dcache + subsystem takes this approach -- updates are guarded + by a global lock, limiting their rate. + + c. Trusted update -- if updates can only be done manually by + superuser or some other trusted user, then it might not + be necessary to automatically limit them. The theory + here is that superuser already has lots of ways to crash + the machine. + + d. Use call_rcu_bh() rather than call_rcu(), in order to take + advantage of call_rcu_bh()'s faster grace periods. + + e. Periodically invoke synchronize_rcu(), permitting a limited + number of updates per grace period. 9. All RCU list-traversal primitives, which include list_for_each_rcu(), list_for_each_entry_rcu(), |