[PATCH V2 0/7] rcu: introduce percpu rcu_preempt_depth

[Lai Jiangshan <laijs@linux.alibaba.com>]

Rather than using the generic version of preempt_count, x86 uses
a special version of preempt_count implementation that take advantage
of x86 (single instruction to access/add/decl/flip-bits of percpu
counter). It makes the preempt_count operations really cheap.

For x86, rcu_preempt_depth can also take the same advantage by
using the same technique.

After the patchset:
 - No function call when using rcu_read_[un]lock().
     It is minor improvement, other arch can also achieve it via
     moving ->rcu_read_lock_nesting and ->rcu_read_unlock_special
     to thread_info, but inlined rcu_read_[un]lock() generates
     more instructions and footprint in other arch generally.
 - Only single instruction for rcu_read_lock().
 - Only 2 instructions for the fast path of rcu_read_unlock().

Patch4 simplifies rcu_read_unlock() by avoid using negative
->rcu_read_lock_nesting, Patch7 introduces the percpu rcu_preempt_depth.
Other patches are for preparation.

changed from v1:
  drop patch1/2 of the v1
  drop merged patches

  Using special.b.deferred_qs to avoid wakeup in v1 is changed to using
  preempt_count. And special.b.deferred_qs is removed.

Lai Jiangshan (7):
  rcu: use preempt_count to test whether scheduler locks is held
  rcu: cleanup rcu_preempt_deferred_qs()
  rcu: remove useless special.b.deferred_qs
  rcu: don't use negative ->rcu_read_lock_nesting
  rcu: wrap usages of rcu_read_lock_nesting
  rcu: clear the special.b.need_qs in rcu_note_context_switch()
  x86,rcu: use percpu rcu_preempt_depth

 arch/x86/Kconfig                         |   2 +
 arch/x86/include/asm/rcu_preempt_depth.h |  87 +++++++++++++++++++
 arch/x86/kernel/cpu/common.c             |   7 ++
 arch/x86/kernel/process_32.c             |   2 +
 arch/x86/kernel/process_64.c             |   2 +
 include/linux/rcupdate.h                 |  24 ++++++
 include/linux/sched.h                    |   2 +-
 init/init_task.c                         |   2 +-
 kernel/fork.c                            |   2 +-
 kernel/rcu/Kconfig                       |   3 +
 kernel/rcu/tree_exp.h                    |  35 ++------
 kernel/rcu/tree_plugin.h                 | 101 ++++++++++++++---------
 12 files changed, 196 insertions(+), 73 deletions(-)
 create mode 100644 arch/x86/include/asm/rcu_preempt_depth.h

-- 
2.20.1