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Subject: Re: [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt_depth
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        "Paul E. McKenney" <paulmck@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>,
        Andi Kleen <ak@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Kees Cook <keescook@chromium.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Babu Moger <Babu.Moger@amd.com>,
        Rik van Riel <riel@surriel.com>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Jann Horn <jannh@google.com>,
        David Windsor <dwindsor@gmail.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Yuyang Du <duyuyang@gmail.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Richard Guy Briggs <rgb@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Michal Hocko <mhocko@suse.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "Dmitry V. Levin" <ldv@altlinux.org>, rcu@vger.kernel.org
References: <20191102124559.1135-1-laijs@linux.alibaba.com>
 <20191102124559.1135-8-laijs@linux.alibaba.com>
 <20191104092519.nukaz5qmgiskzafi@linutronix.de>
From:   Lai Jiangshan <laijs@linux.alibaba.com>
Message-ID: <4878ccfd-7a4e-4f84-9bc3-1d477e077587@linux.alibaba.com>
Date:   Mon, 4 Nov 2019 19:41:20 +0800
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On 2019/11/4 5:25 下午, Sebastian Andrzej Siewior wrote:
> 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?

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.

> 
>> 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.

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


         raw_cpu_add_4(__rcu_preempt_depth, 1);
      d8f:       65 ff 05 00 00 00 00    incl   %gs:0x0(%rip)        # 
d96 <work_busy+0x16>

......


         return GEN_UNARY_RMWcc("decl", __rcu_preempt_depth, e, 
__percpu_arg([var]));
      dd8:       65 ff 0d 00 00 00 00    decl   %gs:0x0(%rip)        # 
ddf <work_busy+0x5f>
         if (unlikely(rcu_preempt_depth_dec_and_test()))
      ddf:       74 26                   je     e07 <work_busy+0x87>

......

                 rcu_read_unlock_special();
      e07:       e8 00 00 00 00          callq  e0c <work_busy+0x8c>

> 
> Boris pointed one thing, there is also DEFINE_PERCPU_RCU_PREEMP_DEPTH.
> 

Thanks for pointing out.

Best regards
Lai