[PATCH v4 tip/core/rcu 0/11] RCU-tasks implementation

[Paul E. McKenney]

Hello!

This series provides v4 of a prototype of an RCU-tasks implementation,
which has been requested to assist with tramopoline removal.  This flavor
of RCU is task-based rather than CPU-based, and has voluntary context
switch, usermode execution, and the idle loops as its only quiescent
states.  This selection of quiescent states ensures that at the end
of a grace period, there will no longer be any tasks depending on a
trampoline that was removed before the beginning of that grace period.
This works because such trampolines do not contain function calls,
do not contain voluntary context switches, do not switch to usermode,
and do not switch to idle.

The patches in this series are as follows:

1.	Adds the basic call_rcu_tasks() functionality.

2.	Provides cond_resched_rcu_qs() to force quiescent states, including
	RCU-tasks quiescent states, in long loops.

3.	Adds synchronous APIs: synchronize_rcu_tasks() and
	rcu_barrier_tasks().

4.	Handle the possibility of tasks being preempted for extended
	periods of time after being removed from the task list.

5.	Adds GPL exports for the above APIs, courtesy of Steven Rostedt.

6.	Adds rcutorture tests for RCU-tasks.

7.	Adds RCU-tasks test cases to rcutorture scripting.

8.	Adds stall-warning checks for RCU-tasks.

9.	Improves RCU-tasks energy efficiency by replacing polling with
	wait/wakeup.

10.	Document RCU-tasks stall-warning messages.

11.	Defer rcu_tasks_kthread() creation until first call_rcu_tasks()
	to avoid populating systems with unneeded kthreads.

Changes from v3:

o	Add do_exit() SRCU hooks to handle tasks being preempted after
	having removed themselves from the task list.  The need for this
	was pointed out by Oleg Nesterov, and the implmentation suggested
	by Lai Jiangshan.

o	Create rcu_tasks_kthread only if call_rcu_tasks() is invoked.

Changes from v2:

o	Use get_task_struct() instead of do_exit() hooks to synchronize
	with exiting tasks, as suggested by Lai Jiangshan.

o	Add checks of ->on_rq to the grace-period-wait polling, again
	as suggested by Lai Jiangshan.

o	Repositioned synchronize_sched() calls and improved their
	comments.

Changes from v1:

o	The lockdep issue with list locking was finessed by ditching
	list locking in favor of having the list manipulated by a single
	kthread.  This change trimmed about 150 highly concurrent lines
	from the implementation.

o	Get rid of the scheduler hooks in favor of polling the
	per-task count of voluntary context switches, in response
	to Peter Zijlstra's concerns about scheduler overhead.

o	Passes more aggressive rcutorture runs, which indicates that
	an increase in rcutorture's aggression is called for.

o	Handled review comments from Peter Zijlstra, Lai Jiangshan,
	Frederic Weisbecker, and Oleg Nesterov.

o	Added RCU-tasks stall-warning documentation.

Remaining issues include:

o	It is not clear that trampolines in functions called from the
	idle loop are correctly handled.  Or if anyone cares about
	trampolines in functions called from the idle loop.

o	The current implementation does not yet recognize tasks that start
	out executing is usermode.  Instead, it waits for the next
	scheduling-clock tick to note them.

o	As a result, the current implementation does not handle nohz_full=
	CPUs executing tasks running in usermode.  There are a couple of
	possible fixes under consideration.

o	If a task is preempted while executing in usermode, the RCU-tasks
	grace period will not end until that task resumes.  (Is there
	some reasonable way to determine that a given preempted task
	was preempted from usermode execution?)

o	More about RCU-tasks needs to be added to Documentation/RCU.

o	There are probably still bugs.

							Thanx, Paul

------------------------------------------------------------------------

 b/Documentation/RCU/stallwarn.txt                             |   33 -
 b/Documentation/kernel-parameters.txt                         |    5 
 b/fs/file.c                                                   |    2 
 b/include/linux/init_task.h                                   |    9 
 b/include/linux/rcupdate.h                                    |   55 +
 b/include/linux/sched.h                                       |   23 
 b/init/Kconfig                                                |   10 
 b/kernel/exit.c                                               |    3 
 b/kernel/rcu/rcutorture.c                                     |   54 +
 b/kernel/rcu/tiny.c                                           |    2 
 b/kernel/rcu/tree.c                                           |   14 
 b/kernel/rcu/tree_plugin.h                                    |    2 
 b/kernel/rcu/update.c                                         |  322 +++++++++-
 b/mm/mlock.c                                                  |    2 
 b/tools/testing/selftests/rcutorture/configs/rcu/TASKS01      |    7 
 b/tools/testing/selftests/rcutorture/configs/rcu/TASKS01.boot |    1 
 b/tools/testing/selftests/rcutorture/configs/rcu/TASKS02      |    6 
 b/tools/testing/selftests/rcutorture/configs/rcu/TASKS02.boot |    1 
 18 files changed, 508 insertions(+), 43 deletions(-)

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