linux-kernel - Re: [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt

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Date:   Mon, 4 Nov 2019 10:25:19 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Kees Cook <keescook@...omium.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Babu Moger <Babu.Moger@....com>,
        Rik van Riel <riel@...riel.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Jann Horn <jannh@...gle.com>,
        David Windsor <dwindsor@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Yuyang Du <duyuyang@...il.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Richard Guy Briggs <rgb@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Dmitry V. Levin" <ldv@...linux.org>, rcu@...r.kernel.org
Subject: Re: [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt_depth

On 2019-11-02 12:45:59 [+0000], Lai Jiangshan wrote:
> Convert x86 to use a per-cpu rcu_preempt_depth. The reason for doing so
> is that accessing per-cpu variables is a lot cheaper than accessing
> task_struct or thread_info variables.

Is there a benchmark saying how much we gain from this?

> We need to save/restore the actual rcu_preempt_depth when switch.
> We also place the per-cpu rcu_preempt_depth close to __preempt_count
> and current_task variable.
> 
> Using the idea of per-cpu __preempt_count.
> 
> No function call when using rcu_read_[un]lock().
> Single instruction for rcu_read_lock().
> 2 instructions for fast path of rcu_read_unlock().

I think these were not inlined due to the header requirements.

Boris pointed one thing, there is also DEFINE_PERCPU_RCU_PREEMP_DEPTH.

Sebastian