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

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Date:	Wed, 21 Aug 2013 13:59:29 +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.


If we still doubt about it, but can not find a suitable way to fix it
(neither of us are familiar with it).

Is it suitable to use BUG_ON() for it (the diff may like below) ?

-------------------------------diff begin-------------------------------

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index dbf74b5..1d02659 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2728,6 +2728,7 @@ static int rcu_cpu_has_callbacks(int cpu, bool *all_lazy)
 		if (rdp->nxtlist)
 			hc = true;
 	}
+	BUG_ON(!hc && !al);
 	if (all_lazy)
 		*all_lazy = al;
 	return hc;

-------------------------------diff end---------------------------------

Thanks.


On 08/20/2013 12:45 PM, Chen Gang wrote:
> 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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