Re: [PATCH] percpu: explain quick paths in pcpu_[de]populate_chunk()

[Cong Wang]

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@xxxxxxxxxx>

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);