[PATCH v3 tip/core/rcu 0/9

Paul E. McKenney
2014-07-31T21:54:53Z

Hello!

This series provides v3 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. Adds GPL exports for the above APIs, courtesy of Steven Rostedt.

5. Adds rcutorture tests for RCU-tasks.

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

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

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

9. Document RCU-tasks stall-warning messages.

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 This version creates rcu_tasks_kthread() even if there never will
be any uses, which is expected to be the common case. A future
version might create rcu_tasks_kthread() on demand, as suggested
off-list by Josh Triplett.

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 | 52 +
b/include/linux/sched.h | 23
b/init/Kconfig | 10
b/kernel/rcu/rcutorture.c | 44 +
b/kernel/rcu/tiny.c | 2
b/kernel/rcu/tree.c | 14
b/kernel/rcu/tree_plugin.h | 2
b/kernel/rcu/update.c | 292 +++++++++-
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
17 files changed, 460 insertions(+), 45 deletions(-)