linux/kernel/rcutree.c, branch v3.8

context_tracking: New context tracking susbsystem

2012-11-30T19:40:07Z

Create a new subsystem that probes on kernel boundaries to keep track of the transitions between level contexts with two basic initial contexts: user or kernel. This is an abstraction of some RCU code that use such tracking to implement its userspace extended quiescent state. We need to pull this up from RCU into this new level of indirection because this tracking is also going to be used to implement an "on demand" generic virtual cputime accounting. A necessary step to shutdown the tick while still accounting the cputime. Signed-off-by: Frederic Weisbecker Cc: Andrew Morton Cc: H. Peter Anvin Cc: Ingo Molnar Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Thomas Gleixner Cc: Li Zhong Cc: Gilad Ben-Yossef Reviewed-by: Steven Rostedt [ paulmck: fix whitespace error and email address. ] Signed-off-by: Paul E. McKenney

rcu: Add callback-free CPUs

2012-11-16T18:05:56Z

RCU callback execution can add significant OS jitter and also can degrade both scheduling latency and, in asymmetric multiprocessors, energy efficiency. This commit therefore adds the ability for selected CPUs ("rcu_nocbs=" boot parameter) to have their callbacks offloaded to kthreads. If the "rcu_nocb_poll" boot parameter is also specified, these kthreads will do polling, removing the need for the offloaded CPUs to do wakeups. At least one CPU must be doing normal callback processing: currently CPU 0 cannot be selected as a no-CBs CPU. In addition, attempts to offline the last normal-CBs CPU will fail. This feature was inspired by Jim Houston's and Joe Korty's JRCU, and this commit includes fixes to problems located by Fengguang Wu's kbuild test robot. [ paulmck: Added gfp.h include file as suggested by Fengguang Wu. ] Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

Merge branches 'urgent.2012.10.27a', 'doc.2012.11.16a', 'fixes.2012.11.13a', 'srcu.2012.10.27a', 'stall.2012.11.13a', 'tracing.2012.11.08a' and 'idle.2012.10.24a' into HEAD

2012-11-16T17:59:58Z

urgent.2012.10.27a: Fix for RCU user-mode transition (already in -tip). doc.2012.11.08a: Documentation updates, most notably codifying the memory-barrier guarantees inherent to grace periods. fixes.2012.11.13a: Miscellaneous fixes. srcu.2012.10.27a: Allow statically allocated and initialized srcu_struct structures (courtesy of Lai Jiangshan). stall.2012.11.13a: Add more diagnostic information to RCU CPU stall warnings, also decrease from 60 seconds to 21 seconds. hotplug.2012.11.08a: Minor updates to CPU hotplug handling. tracing.2012.11.08a: Improved debugfs tracing, courtesy of Michael Wang. idle.2012.10.24a: Updates to RCU idle/adaptive-idle handling, including a boot parameter that maps normal grace periods to expedited. Resolved conflict in kernel/rcutree.c due to side-by-side change.

rcu: Clarify memory-ordering properties of grace-period primitives

2012-11-13T22:08:23Z

This commit explicitly states the memory-ordering properties of the RCU grace-period primitives. Although these properties were in some sense implied by the fundmental property of RCU ("a grace period must wait for all pre-existing RCU read-side critical sections to complete"), stating it explicitly will be a great labor-saving device. Reported-by: Oleg Nesterov Signed-off-by: Paul E. McKenney Reviewed-by: Oleg Nesterov

rcu: Fix batch-limit size problem

2012-11-13T22:07:57Z

Commit 29c00b4a1d9e27 (rcu: Add event-tracing for RCU callback invocation) added a regression in rcu_do_batch() Under stress, RCU is supposed to allow to process all items in queue, instead of a batch of 10 items (blimit), but an integer overflow makes the effective limit being 1. So, unless there is frequent idle periods (during which RCU ignores batch limits), RCU can be forced into a state where it cannot keep up with the callback-generation rate, eventually resulting in OOM. This commit therefore converts a few variables in rcu_do_batch() from int to long to fix this problem, along with the module parameters controlling the batch limits. Signed-off-by: Eric Dumazet Signed-off-by: Paul E. McKenney Cc: # 3.2 +

rcu: Fix tracing formatting

2012-11-08T19:55:30Z

The rcu_state structure's ->completed field is unsigned long, so this commit adjusts show_one_rcugp()'s printf() format to suit. Also add the required ACCESS_ONCE() directives while we are in this function. Signed-off-by: Paul E. McKenney

rcu: Instrument synchronize_rcu_expedited() for debugfs tracing

2012-11-08T19:50:13Z

This commit adds the counters to rcu_state and updates them in synchronize_rcu_expedited() to provide the data needed for debugfs tracing. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Move synchronize_sched_expedited() state to rcu_state

2012-11-08T19:50:12Z

Tracing (debugfs) of expedited RCU primitives is required, which in turn requires that the relevant data be located where the tracing code can find it, not in its current static global variables in kernel/rcutree.c. This commit therefore moves sync_sched_expedited_started and sync_sched_expedited_done to the rcu_state structure, as fields ->expedited_start and ->expedited_done, respectively. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Avoid counter wrap in synchronize_sched_expedited()

2012-11-08T19:50:12Z

There is a counter scheme similar to ticket locking that synchronize_sched_expedited() uses to service multiple concurrent callers with the same expedited grace period. Upon entry, a sync_sched_expedited_started variable is atomically incremented, and upon completion of a expedited grace period a separate sync_sched_expedited_done variable is atomically incremented. However, if a synchronize_sched_expedited() is delayed while in try_stop_cpus(), concurrent invocations will increment the sync_sched_expedited_started counter, which will eventually overflow. If the original synchronize_sched_expedited() resumes execution just as the counter overflows, a concurrent invocation could incorrectly conclude that an expedited grace period elapsed in zero time, which would be bad. One could rely on counter size to prevent this from happening in practice, but the goal is to formally validate this code, so it needs to be fixed anyway. This commit therefore checks the gap between the two counters before incrementing sync_sched_expedited_started, and if the gap is too large, does a normal grace period instead. Overflow is thus only possible if there are more than about 3.5 billion threads on 32-bit systems, which can be excluded until such time as task_struct fits into a single byte and 4G/4G patches are accepted into mainline. It is also easy to encode this limitation into mechanical theorem provers. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney

rcu: Rename ->onofflock to ->orphan_lock

2012-11-08T19:50:11Z

The ->onofflock field in the rcu_state structure at one time synchronized CPU-hotplug operations for RCU. However, its scope has decreased over time so that it now only protects the lists of orphaned RCU callbacks. This commit therefore renames it to ->orphan_lock to reflect its current use. Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney