linux-kernel - Re: [PATCH] percpu: explain quick paths in pcpu_[de]populate

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Message-ID: <4B14AC35.3020700@redhat.com>
Date:	Tue, 01 Dec 2009 13:40:05 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] percpu: explain quick paths in pcpu_[de]populate_chunk()

Tejun Heo wrote:
> On 12/01/2009 02:00 PM, Tejun Heo wrote:
>> I thought about that but didn't want to open code the special and
>> fairly complex loop construct used there.  To me, it seemed using the
>> same loop construct would be much less error-prone than open coding
>> the loop mostly because those two special cases are the only place
>> where that is necessary.  Maybe we can add pcpu_first_[un]pop_region()
>> macros and use them there but is the first iteration check that bad
>> even with sufficient explanations?
> 
> So, something like the following.

Thanks for working on this.

> 
> #define pcpu_first_unpop_region(chunk, rs, re, start, end)	do {	\
> 	(rs) = (start);							\
> 	pcpu_next_unpop((chunk), &(rs), &(re), (end));			\
> } while (0)
> 
> #define pcpu_for_each_unpop_region(chunk, rs, re, start, end)		\
> 	for (pcpu_first_unpop_region(chunk, rs, re, start, end);	\
> 	     (rs) < (re);						\
> 	     (rs) = (re) + 1, pcpu_next_unpop((chunk), &(rs), &(re), (end)))
> 
> #define pcpu_first_pop_region(chunk, rs, re, start, end)	do {	\
> 	(rs) = (start);							\
> 	pcpu_next_pop((chunk), &(rs), &(re), (end));			\
> } while (0)
> 
> #define pcpu_for_each_pop_region(chunk, rs, re, start, end)		\
> 	for (pcpu_first_pop_region(chunk, rs, re, start, end);		\
> 	     (rs) < (re);						\
> 	     (rs) = (re) + 1, pcpu_next_pop((chunk), &(rs), &(re), (end)))
> 
> It might be better to make these proper functions which take pointers
> but that makes the only two interfaces for region iterators disagree
> about how they take parameters.
> 
> So, I don't know.  The first iteration only loop looks a bit unusual
> for sure but it isn't something conceptually convoluted.

Now this seems to be better. So with this change, we can do:

pcpu_first_pop_region(chunk, rs, re, start, end);
if (rs < re && ...)
    return;

Right?

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