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    SubjectRe: [PATCH V2 0/7] rcu: introduce percpu rcu_preempt_depth
    On Sat, Nov 02, 2019 at 12:45:52PM +0000, Lai Jiangshan wrote:
    > 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

    I am getting on a plane shortly, so what I have done is start a longish
    rcutorture test on these. I will take a look at them later on.

    And I presume that answers to my questions on some of the patches in
    this series will be forthcoming. ;-)

    Thanx, Paul

    > 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
    >

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