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Message-Id: <1250276525.10725.6749.camel@nimitz>
Date: Fri, 14 Aug 2009 12:02:05 -0700
From: Dave Hansen <dave@...ux.vnet.ibm.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
containers@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, menage@...gle.com,
vda.linux@...glemail.com, mikew@...gle.com, lizf@...fujitsu.com,
xiyou.wangcong@...il.com, kamezawa.hiroyu@...fujitsu.com,
hpa@...or.com, bblum@...gle.com
Subject: Re: [RFC][PATCH] flexible array implementation v4
On Fri, 2009-08-14 at 10:56 -0700, Dan Williams wrote:
> On Fri, Jul 24, 2009 at 4:38 PM, Andrew Morton<akpm@...ux-foundation.org> wrote:
> > On Thu, 23 Jul 2009 08:26:47 -0700
> > Dave Hansen <dave@...ux.vnet.ibm.com> wrote:
> >
> >> linux-2.6.git-dave/include/linux/flex_array.h | 46 ++++
> >> linux-2.6.git-dave/lib/Makefile | 2
> >> linux-2.6.git-dave/lib/flex_array.c | 269 ++++++++++++++++++++++++++
> >
> > I haven't looked at this lately but I merged it.
> >
> > As it's obviously non-injurious, we could slip it into 2.6.31 for
> > convenience's sake but without any callers that's just runtime bloat.
> > So I guess we wait for some additional callers to come along and prove
> > its worth.
>
> I am considering using this for managing the descriptor ring in a
> device driver, but I am concerned with the overhead of the divides.
> Would it be acceptable to round the element size to the next
> power-of-2 value so we can replace all the divides with shifts?
I think we have two options. We can programatically determine and store
the shift, like this (during allocation):
if (can_shift)
fa->element_size = -log2(element_size);
else
fa->element_size = element_size;
Then, when dividing, we look for the "negative" offset size and do:
if (element_size < 0)
index = foo >> fa->element_size;
else
index = foo / fa->element_size;
Or, we can do what I'm doing in this patch. If you want to specify the
index sizes to flex_array_put/get(), there's a flex_array_put_es()
(element size) variant. It should inline enough to let the compiler
take care of this for us and turn the divides into shifts.
I just hacked this up *REALLY* quickly. Not even compile tested. I'll
look at it in some more detail early next week. I'm not even positive
this will work. Just releasing early and often. :)
diff --git a/include/linux/flex_array.h b/include/linux/flex_array.h
index 23c1ec7..9052704 100644
--- a/include/linux/flex_array.h
+++ b/include/linux/flex_array.h
@@ -36,6 +36,47 @@ struct flex_array {
.total_nr_elements = (total), \
} } }
+static inline int __elements_per_part(int element_size)
+{
+ return FLEX_ARRAY_PART_SIZE / element_size;
+}
+
+static int __fa_element_to_part_nr(struct flex_array *fa, int element_size
+ int element_nr)
+{
+ return element_nr / __elements_per_part(element_size);
+}
+
+static int __fa_index_inside_part(struct flex_array *fa, int element_size,
+ int element_nr)
+{
+ return element_nr % __elements_per_part(element_size);
+}
+
+void *flex_array_get_es(struct flex_array *fa, int element_nr, int element_size);
+{
+ int part_nr = __fa_element_to_part_nr(fa, element_nr, element_size);
+ int index_inside_part = index_inside_part(fa, element_nr);
+
+ if (element_nr >= fa->total_nr_elements)
+ return -ENOSPC;
+
+ return flex_array_get_precalc(fa, part_nr, index_inside_part);
+}
+
+int flex_array_put_es(struct flex_array *fa, int element_nr, int element_size,
+ void *src, gfp_t flags);
+{
+ int part_nr = __fa_element_to_part_nr(fa, element_nr, element_size);
+ int index_inside = index_inside_part(fa, element_nr);
+
+ if (element_nr >= fa->total_nr_elements)
+ return NULL;
+
+ return flex_array_put_precalc(fa, part_nr, index_inside, src, flags);
+}
+
+
struct flex_array *flex_array_alloc(int element_size, int total, gfp_t flags);
int flex_array_prealloc(struct flex_array *fa, int start, int end, gfp_t flags);
void flex_array_free(struct flex_array *fa);
diff --git a/lib/flex_array.c b/lib/flex_array.c
index 08f1636..0b2030d 100644
--- a/lib/flex_array.c
+++ b/lib/flex_array.c
@@ -115,11 +115,6 @@ struct flex_array *flex_array_alloc(int element_size, int total, gfp_t flags)
return ret;
}
-static int fa_element_to_part_nr(struct flex_array *fa, int element_nr)
-{
- return element_nr / __elements_per_part(fa->element_size);
-}
-
/**
* flex_array_free_parts - just free the second-level pages
* @src: address of data to copy into the array
@@ -153,7 +148,7 @@ static int fa_index_inside_part(struct flex_array *fa, int element_nr)
static int index_inside_part(struct flex_array *fa, int element_nr)
{
- int part_offset = fa_index_inside_part(fa, element_nr);
+ int part_offset = fa_index_inside_part(fa, fa->element_size, element_nr);
return part_offset * fa->element_size;
}
@@ -176,6 +171,17 @@ __fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
return part;
}
+
+static int fa_element_to_part_nr(struct flex_array *fa, int element_nr)
+{
+ return __fa_element_to_part_nr(fa, fa->element_size, element_nr);
+}
+
+static int fa_index_inside_part(struct flex_array *fa, int element_nr)
+{
+ return __fa_index_inside_part(fa, fa->element_size, element_nr);
+}
+
/**
* flex_array_put - copy data into the array at @element_nr
* @src: address of data to copy into the array
@@ -188,9 +194,17 @@ __fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
*
* Locking must be provided by the caller.
*/
-int flex_array_put(struct flex_array *fa, int element_nr, void *src, gfp_t flags)
+int flex_array_put(struct flex_array *fa, int element_nr, void *src,
+ gfp_t flags)
{
int part_nr = fa_element_to_part_nr(fa, element_nr);
+ int index_inside = index_inside_part(fa, element_nr);
+
+ return flex_array_put_precalc(fa, part_nr, index_inside, src, flags);
+}
+int flex_array_put_precalc(struct flex_array *fa, int part_nr,
+ int index_inside_part, void *src, gfp_t flags)
+{
struct flex_array_part *part;
void *dst;
@@ -253,15 +267,23 @@ int flex_array_prealloc(struct flex_array *fa, int start, int end, gfp_t flags)
void *flex_array_get(struct flex_array *fa, int element_nr)
{
int part_nr = fa_element_to_part_nr(fa, element_nr);
- struct flex_array_part *part;
+ int index_inside_part = index_inside_part(fa, element_nr);
if (element_nr >= fa->total_nr_elements)
return NULL;
+
+ return flex_array_get_precalc(fa, part_nr, index_inside_part);
+}
+void *flex_array_get_precalc(struct flex_array *fa, int part_nr,
+ int index_inside_part)
+{
+ struct flex_array_part *part;
+
if (!fa->parts[part_nr])
return NULL;
if (elements_fit_in_base(fa))
part = (struct flex_array_part *)&fa->parts[0];
else
part = fa->parts[part_nr];
- return &part->elements[index_inside_part(fa, element_nr)];
+ return &part->elements[index_inside_part];
}
-- Dave
--
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