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Date:   Sat, 2 Nov 2019 08:05:45 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Lai Jiangshan <laijs@linux.alibaba.com>
Cc:     linux-kernel@vger.kernel.org,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org
Subject: Re: [PATCH V2 0/7] rcu: introduce percpu rcu_preempt_depth
Message-ID: <20191102150545.GQ20975@paulmck-ThinkPad-P72>
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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
>