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

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Date:   Mon, 4 Nov 2019 13:09:39 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org,
        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 2019-11-04 19:41:20 [+0800], Lai Jiangshan wrote:
> > Is there a benchmark saying how much we gain from this?
> 
> Hello
> 
> Maybe I can write a tight loop for testing, but I don't
> think anyone will be interesting in it.
> 
> I'm also trying to find some good real tests. I need
> some suggestions here.

There is rcutorture but I don't know how much of performance test this
is, Paul would know.

A micro benchmark is one thing. Any visible changes in userland to
workloads like building a kernel or hackbench? 

I don't argue that incrementing a per-CPU variable is more efficient
than reading a per-CPU variable, adding an offset and then incrementing
it. I was just curious to see if there are any numbers on it.

> > > 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.
> 
> objdump -D -S kernel/workqueue.o shows (selected fractions):

That was not what I meant. To inline current rcu_read_lock() would mean
to include definition for struct task_struct (and everything down the
road) in the rcu headers which isn't working.

> Best regards
> Lai

Sebastian