On Thu, Sep 14, 2017 at 09:56:56AM +0800, Ming Lei wrote:
> On Wed, Sep 13, 2017 at 11:37:20AM -0700, Omar Sandoval wrote:
> > On Mon, Sep 11, 2017 at 12:08:29PM +0800, Ming Lei wrote:
> > > On Sun, Sep 10, 2017 at 10:20:27AM -0700, Omar Sandoval wrote:
> >
> > [snip]
> >
> > > > What I mean is that you keep the same initialization above, but instead of
> > > > depth += nr
> > > > you do
> > > > depth = min_t(unsigned int, word->depth, sb->depth - scanned);
> > > > because like I said, the reasoning about why `+= nr` is okay in the
> > > > `sb->depth - scanned` case is subtle.
> > > >
> > > > And maybe even replace the
> > > > scanned += depth;
> > > > with
> > > > scanned += min_t(unsigned int, word->depth - nr,
> > > > sb->depth - scanned);
> > > > I.e., don't reuse the depth local variable for two different things. I'm
> > > > nitpicking here but this code is tricky enough as it is.
> > >
> > > It wasn't reused in old version, just for saving one local variable, and
> > > one extra min_t().
> > >
> > > Yeah, I admit it isn't clean enough.
> > >
> > > >
> > > > For completeness, I mean this exactly:
> > > >
> > > > while (1) {
> > > > struct sbitmap_word *word = &sb->map[index];
> > > > unsigned int depth;
> > > >
> > > > scanned += min_t(unsigned int, word->depth - nr,
> > > > sb->depth - scanned);
> > > > if (!word->word)
> > > > goto next;
> > > >
> > > > depth = min_t(unsigned int, word->depth, sb->depth - scanned);
> > >
> > > two min_t and a little code duplication.
> >
> > They're similar but they represent different things, so I think trying
> > to deduplicate this code just makes it more confusing. If performance is
> > your concern, I'd be really surprised if there's a noticable difference.
>
> No only one extra min_t(), also it isn't easy to read the code, since
> only in the first scan that 'depth' isn't same with 'depth', that is
> why I set the 1st 'scan' outside of the loop, then we can update 'scan'
> with 'depth' in every loop. People will be easy to follow the
> meaning.
>
> >
> > As a side note, I also realized that this code doesn't handle the
> > sb->depth == 0 case. We should change the while (1) to
> > while (scanned < sb->depth) and remove the
> > if (scanned >= sb->depth) break;
>
> In the attached patch, I remember that the zero depth case is
> addressed by:
>
> if (start >= sb->depth)
> return;
>
> which is required since 'start' parameter is introduced in
> this patch.
I think the better way to handle this is
if (start >= sb->depth)
start = 0;
Since the sbitmap may have gotten resized since the last time the user
called this and cached their start value.
> >
> > > > off = index << sb->shift;
> > > > while (1) {
> > > > nr = find_next_bit(&word->word, depth, nr);
> > > > if (nr >= depth)
> > > > break;
> > > >
> > > > if (!fn(sb, off + nr, data))
> > > > return;
> > > >
> > > > nr++;
> > > > }
> > > > next:
> > > > if (scanned >= sb->depth)
> > > > break;
> > > > nr = 0;
> > > > if (++index >= sb->map_nr)
> > > > index = 0;
> > > > }
> > >
> > > The following patch switches to do{}while and handles the
> > > 1st scan outside of the loop, then it should be clean
> > > enough(no two min_t()), so how about this one?
> >
> > I find this one subtler and harder to follow. The less it looks like the
> > typical loop pattern, the longer someone reading the code has to reason
> > about it.
>
> Looks using 'depth' to update 'scanned' is easier to follow, than
> two min_t(), since it will make people easy to understand the relation
> between the two, then understand the whole code.
Honestly I prefer your original patch with a comment on depth += nr. I'd
be happy with the following incremental patch on top of your original v4
patch.
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index 2329b9e1a0e2..8d747048ae4f 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -218,7 +218,7 @@ typedef bool (*sb_for_each_fn)(struct sbitmap *, unsigned int, void *);
/**
* sbitmap_for_each_set() - Iterate over each set bit in a &struct sbitmap.
- * @off: Where to start the iteration
+ * @off: Where to start the iteration.
* @sb: Bitmap to iterate over.
* @fn: Callback. Should return true to continue or false to break early.
* @data: Pointer to pass to callback.
@@ -230,11 +230,16 @@ static inline void __sbitmap_for_each_set(struct sbitmap *sb,
unsigned int off,
sb_for_each_fn fn, void *data)
{
- unsigned int index = SB_NR_TO_INDEX(sb, off);
- unsigned int nr = SB_NR_TO_BIT(sb, off);
+ unsigned int index;
+ unsigned int nr;
unsigned int scanned = 0;
- while (1) {
+ if (off >= sb->depth)
+ off = 0;
+ index = SB_NR_TO_INDEX(sb, off);
+ nr = SB_NR_TO_BIT(sb, off);
+
+ while (scanned < sb->depth) {
struct sbitmap_word *word = &sb->map[index];
unsigned int depth = min_t(unsigned int, word->depth - nr,
sb->depth - scanned);
@@ -243,6 +248,11 @@ static inline void __sbitmap_for_each_set(struct sbitmap *sb,
if (!word->word)
goto next;
+ /*
+ * On the first iteration of the outer loop, we need to add the
+ * bit offset back to the size of the word for find_next_bit().
+ * On all other iterations, nr is zero, so this is a noop.
+ */
depth += nr;
off = index << sb->shift;
while (1) {
@@ -254,9 +264,7 @@ static inline void __sbitmap_for_each_set(struct sbitmap *sb,
nr++;
}
- next:
- if (scanned >= sb->depth)
- break;
+next:
nr = 0;
if (++index >= sb->map_nr)
index = 0;
@@ -268,9 +276,6 @@ static inline void __sbitmap_for_each_set(struct sbitmap *sb,
* @sb: Bitmap to iterate over.
* @fn: Callback. Should return true to continue or false to break early.
* @data: Pointer to pass to callback.
- *
- * This is inline even though it's non-trivial so that the function calls to the
- * callback will hopefully get optimized away.
*/
static inline void sbitmap_for_each_set(struct sbitmap *sb, sb_for_each_fn fn,
void *data)
