linux-kernel - Re: [PATCH] kernel/rcutree.c: deem to be lazy if there are no callbacks.

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Date:	Tue, 20 Aug 2013 12:45:27 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	dipankar@...ibm.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/rcutree.c: deem to be lazy if there are no callbacks.

On 08/20/2013 12:43 PM, Chen Gang wrote:
> On 08/20/2013 12:18 PM, Paul E. McKenney wrote:
>> On Tue, Aug 20, 2013 at 11:51:23AM +0800, Chen Gang wrote:
>>>
>>>
>>> If 'hc' is false, 'al' will never be false, either (only need check
>>> "irdp->qlen != rdp->qlen_lazy' when 'rdp->nxtlist' existance).
>>>
>>> Recommend to improve the related code, like the diff below.
>>
>> Are you sure that this represents an improvement?  If so, why?
>>
> 
> If 'hc' and 'al' really has relationships, better to let 'C code'
> express it, that will make the code clearer.
> 
>> Or to put it another way, I see a patch that increases the size of the
>> kernel by three lines.  What is the corresponding benefit given common
>> kernel workloads?
>>
> 
> For 'al', need not check for each looping, and for 'hc', may save the
> useless looping (so it can make performance better).
> 
> For C code, it really increases 3 lines, but may not for assembly code
> (excuse me, I am not check it, I think it is not important, although it
> is easy to give a comparing for binary).
> 

Oh, sorry, I mean: only for our case, "it is not important".


>> 							Thanx, Paul
>>
>>> ----------------------------------diff begin------------------------------------
>>>
>>> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
>>> index 5b53a89..421caf0 100644
>>> --- a/kernel/rcutree.c
>>> +++ b/kernel/rcutree.c
>>> @@ -2719,10 +2719,13 @@ static int rcd'_cpu_has_callbacks(int cpu, bool *all_lazy)
>>>
>>>  	for_each_rcu_flavor(rsp) {
>>>  		rdp = per_cpu_ptr(rsp->rda, cpu);
>>> -		if (rdp->qlen != rdp->qlen_lazy)
>>> -			al = false;
>>> -		if (rdp->nxtlist)
>>> +		if (rdp->nxtlist) {
>>>  			hc = true;
>>> +			if (rdp->qlen != rdp->qlen_lazy) {
>>> +				al = false;
>>> +				break;
>>> +			}
>>> +		}
>>>  	}
>>>  	if (all_lazy)
>>>  		*all_lazy = al;
>>>
>>> ----------------------------------diff end--------------------------------------
>>>
>>>
>>> On 08/20/2013 11:50 AM, Chen Gang wrote:
>>>> According to the comment above rcu_cpu_has_callbacks(): "If there are
>>>> no callbacks, all of them are deemed to be lazy".
>>>>
>>>> So when both 'hc' and 'al' are false, '*all_lazy' should be true, not
>>>> false.
>>>>
>>>>
>>>> Signed-off-by: Chen Gang <gang.chen@...anux.com>
>>>> ---
>>>>  kernel/rcutree.c |    2 +-
>>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
>>>> index 5b53a89..9ee9565 100644
>>>> --- a/kernel/rcutree.c
>>>> +++ b/kernel/rcutree.c
>>>> @@ -2725,7 +2725,7 @@ static int rcu_cpu_has_callbacks(int cpu, bool *all_lazy)
>>>>  			hc = true;
>>>>  	}
>>>>  	if (all_lazy)
>>>> -		*all_lazy = al;
>>>> +		*all_lazy = !hc ? true : al;
>>>>  	return hc;
>>>>  }
>>>>  
>>>>
>>>
>>>
>>> -- 
>>> Chen Gang
>>>
>>
>>
>>
> 
> 


-- 
Chen Gang
--
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