linux-kernel - Re: [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs

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Message-ID: <49c778ff-2187-26fb-1477-bdef6eaf298b@linux.alibaba.com>
Date:   Thu, 31 Oct 2019 23:25:11 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.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>, rcu@...r.kernel.org
Subject: Re: [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs_irqrestore()



On 2019/10/31 9:52 下午, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:07:58AM +0000, Lai Jiangshan wrote:
>> Remove several unneeded return.
>>
>> It doesn't need to return earlier after every code block.
>> The code protects itself and be safe to fall through because
>> every code block has its own condition tests.
>>
>> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
>> ---
>>   kernel/rcu/tree_plugin.h | 14 +-------------
>>   1 file changed, 1 insertion(+), 13 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index 59ef10da1e39..82595db04eec 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -439,19 +439,10 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>>   	 * t->rcu_read_unlock_special cannot change.
>>   	 */
>>   	special = t->rcu_read_unlock_special;
>> -	rdp = this_cpu_ptr(&rcu_data);
>> -	if (!special.s && !rdp->exp_deferred_qs) {
>> -		local_irq_restore(flags);
>> -		return;
>> -	}
> 
> The point of this check is the common case of this function being invoked
> when both fields are zero, avoiding the below redundant store and all the
> extra checks of subfields of special.
> 
> Or are you saying that current compilers figure all this out?

No.

So, I have to keep the first/above return branch.

Any reasons to keep the following 2 return branches?
There is no redundant store and the load for the checks
are hot in the cache if the condition for return is met.

Thanks.
Lai

> 
> 							Thanx, Paul
> 
>>   	t->rcu_read_unlock_special.b.deferred_qs = false;
>>   	if (special.b.need_qs) {
>>   		rcu_qs();
>>   		t->rcu_read_unlock_special.b.need_qs = false;
>> -		if (!t->rcu_read_unlock_special.s && !rdp->exp_deferred_qs) {
>> -			local_irq_restore(flags);
>> -			return;
>> -		}
>>   	}
>>   
>>   	/*
>> @@ -460,12 +451,9 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>>   	 * tasks are handled when removing the task from the
>>   	 * blocked-tasks list below.
>>   	 */
>> +	rdp = this_cpu_ptr(&rcu_data);
>>   	if (rdp->exp_deferred_qs) {
>>   		rcu_report_exp_rdp(rdp);
>> -		if (!t->rcu_read_unlock_special.s) {
>> -			local_irq_restore(flags);
>> -			return;
>> -		}
>>   	}
>>   
>>   	/* Clean up if blocked during RCU read-side critical section. */
>> -- 
>> 2.20.1
>>