Message-ID: <20130821142301.GX29406@linux.vnet.ibm.com>
Date:	Wed, 21 Aug 2013 07:23:01 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Chen Gang <gang.chen@...anux.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 Wed, Aug 21, 2013 at 01:59:29PM +0800, Chen Gang wrote:
> 
> If we still doubt about it, but can not find a suitable way to fix it
> (neither of us are familiar with it).

Well, you have that halfway correct, which some might well argue is an
upward trend from your earlier postings.  I do appreciate your honesty
in saying that you are not familiar with it.  That said, most people
reading these emails will have figured that out by now.

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

Hmmm...  Since my use of questions seems to have confused you, I will
not use questions in this reply.  (The Google search string "quick quiz
site:lwn.net" may help you avoid this confusion in the future.)

And no, the patch below is not appropriate.  Of course, even if it
was appropriate to accept it, I would be unable to do so, as it has
no Signed-of-by.  But I would expect that you already knew that,
given that your first patch (which was also inappropriate) did have
your Signed-off-by.

In addition, I cordially invite you to interpret my questions in the
earlier emails in this thread as quick quizzes, and to contemplate how
you have been doing with your answers.

Don't get me wrong, I do welcome appropriate patches.  In fact, if
you look at RCU's git history, you will see that I frequently accept
patches from a fair number of people.  And if you were willing to
invest some time and thought, you might eventually be able to generate
an appropriate (albeit low priority) patch to this function.  However,
you seem to be motivated to submit small patches with a minimum of
thought and preparation, perhaps because you need to meet some external
or self-imposed quota of accepted patches.  And if you are in fact driven
by a quota that prevents you from taking the time required to carefully
think things through, you are wasting your time with RCU.

Good luck!

							Thanx, Paul

> -------------------------------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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