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SubjectRe: [PATCH] percpu: explain quick paths in pcpu_[de]populate_chunk()
Tejun Heo wrote:
> Hello,
>
> On 12/01/2009 02:40 PM, Cong Wang wrote:
>>> 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?
>
> Yeap, but is that any better? Passing lvalue loop parameters to loop
> constructs is customary. For almost all other cases, it's not, so
>
> pcpu_first_pop_region(chunk, &rs, &re, start, end)
>
> would be better but then we have two similar looking interfaces which
> take different types of parameters. Also, you probably can drop rs <
> re test there but for me it just seems easier to simply check the
> first iteration. If you think it's something worth changing and it
> looks better afterwards, please send a patch.
>

What do you think about the patch below? Untested.

-----------

Signed-off-by: WANG Cong <amwang@redhat.com>

diff --git a/mm/percpu.c b/mm/percpu.c
index 5adfc26..d1da616 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -911,14 +911,12 @@ static void pcpu_depopulate_chunk(struct pcpu_chunk *chunk, int off, int size)
int page_end = PFN_UP(off + size);
struct page **pages;
unsigned long *populated;
- int rs, re;
+ int rs = page_start, re;

/* quick path, check whether it's empty already */
- pcpu_for_each_unpop_region(chunk, rs, re, page_start, page_end) {
- if (rs == page_start && re == page_end)
- return;
- break;
- }
+ pcpu_next_unpop(chunk, &rs, &re, page_end);
+ if (rs == page_start && re == page_end)
+ return;

/* immutable chunks can't be depopulated */
WARN_ON(chunk->immutable);
@@ -966,14 +964,12 @@ static int pcpu_populate_chunk(struct pcpu_chunk *chunk, int off, int size)
struct page **pages;
unsigned long *populated;
unsigned int cpu;
- int rs, re, rc;
+ int rs = page_start, re, rc;

/* quick path, check whether all pages are already there */
- pcpu_for_each_pop_region(chunk, rs, re, page_start, page_end) {
- if (rs == page_start && re == page_end)
- goto clear;
- break;
- }
+ pcpu_next_pop(chunk, &rs, &re, page_end);
+ if (rs == page_start && re == page_end)
+ goto clear;

/* need to allocate and map pages, this chunk can't be immutable */
WARN_ON(chunk->immutable);
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