Merge to Fedora kernel-2.6.18-1.2224_FC5 patched with stable patch-2.6.18.1-vs2.0...

[linux-2.6.git] / Documentation / RCU / checklist.txt

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 49e27cc..1d50cf0 100644 (file)
--- 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(),