diff options
Diffstat (limited to 'Documentation/RCU')
| -rw-r--r-- | Documentation/RCU/checklist.txt | 46 | ||||
| -rw-r--r-- | Documentation/RCU/stallwarn.txt | 18 | ||||
| -rw-r--r-- | Documentation/RCU/trace.txt | 13 |
3 files changed, 69 insertions, 8 deletions
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 790d1a8..0c134f8 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -218,13 +218,22 @@ over a rather long period of time, but improvements are always welcome! 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. + 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. + + One way to stall the updates is to acquire the update-side + mutex. (Don't try this with a spinlock -- other CPUs + spinning on the lock could prevent the grace period + from ever ending.) Another way to stall the updates + is for the updates to use a wrapper function around + the memory allocator, so that this wrapper function + simulates OOM when there is too much memory awaiting an + RCU grace period. There are of course many other + variations on this theme. b. Limiting update rate. For example, if updates occur only once per hour, then no explicit rate limiting is required, @@ -365,3 +374,26 @@ over a rather long period of time, but improvements are always welcome! and the compiler to freely reorder code into and out of RCU read-side critical sections. It is the responsibility of the RCU update-side primitives to deal with this. + +17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and + the __rcu sparse checks to validate your RCU code. These + can help find problems as follows: + + CONFIG_PROVE_RCU: check that accesses to RCU-protected data + structures are carried out under the proper RCU + read-side critical section, while holding the right + combination of locks, or whatever other conditions + are appropriate. + + CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the + same object to call_rcu() (or friends) before an RCU + grace period has elapsed since the last time that you + passed that same object to call_rcu() (or friends). + + __rcu sparse checks: tag the pointer to the RCU-protected data + structure with __rcu, and sparse will warn you if you + access that pointer without the services of one of the + variants of rcu_dereference(). + + These debugging aids can help you find problems that are + otherwise extremely difficult to spot. diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt index 44c6dcc..862c08e 100644 --- a/Documentation/RCU/stallwarn.txt +++ b/Documentation/RCU/stallwarn.txt @@ -80,6 +80,24 @@ o A CPU looping with bottom halves disabled. This condition can o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel without invoking schedule(). +o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might + happen to preempt a low-priority task in the middle of an RCU + read-side critical section. This is especially damaging if + that low-priority task is not permitted to run on any other CPU, + in which case the next RCU grace period can never complete, which + will eventually cause the system to run out of memory and hang. + While the system is in the process of running itself out of + memory, you might see stall-warning messages. + +o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that + is running at a higher priority than the RCU softirq threads. + This will prevent RCU callbacks from ever being invoked, + and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent + RCU grace periods from ever completing. Either way, the + system will eventually run out of memory and hang. In the + CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning + messages. + o A bug in the RCU implementation. o A hardware failure. This is quite unlikely, but has occurred diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt index efd8cc9..a851118 100644 --- a/Documentation/RCU/trace.txt +++ b/Documentation/RCU/trace.txt @@ -125,6 +125,17 @@ o "b" is the batch limit for this CPU. If more than this number of RCU callbacks is ready to invoke, then the remainder will be deferred. +o "ci" is the number of RCU callbacks that have been invoked for + this CPU. Note that ci+ql is the number of callbacks that have + been registered in absence of CPU-hotplug activity. + +o "co" is the number of RCU callbacks that have been orphaned due to + this CPU going offline. + +o "ca" is the number of RCU callbacks that have been adopted due to + other CPUs going offline. Note that ci+co-ca+ql is the number of + RCU callbacks registered on this CPU. + There is also an rcu/rcudata.csv file with the same information in comma-separated-variable spreadsheet format. @@ -180,7 +191,7 @@ o "s" is the "signaled" state that drives force_quiescent_state()'s o "jfq" is the number of jiffies remaining for this grace period before force_quiescent_state() is invoked to help push things - along. Note that CPUs in dyntick-idle mode thoughout the grace + along. Note that CPUs in dyntick-idle mode throughout the grace period will not report on their own, but rather must be check by some other CPU via force_quiescent_state(). |