xfs: improve xfarray quicksort pivot

Now that we have the means to do insertion sorts of small in-memory
subsets of an xfarray, use it to improve the quicksort pivot algorithm
by reading 7 records into memory and finding the median of that.  This
should prevent bad partitioning when a[lo] and a[hi] end up next to each
other in the final sort, which can happen when sorting for cntbt repair
when the free space is extremely fragmented (e.g. generic/176).

This doesn't speed up the average quicksort run by much, but it will
(hopefully) avoid the quadratic time collapse for which quicksort is
famous.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

1 files changed, 14 insertions, 5 deletions

diff --git a/fs/xfs/scrub/xfarray.h b/fs/xfs/scrub/xfarray.h
index 091614e7f683..4ecac01363d9 100644
--- a/fs/xfs/scrub/xfarray.h
+++ b/fs/xfs/scrub/xfarray.h
@@ -62,6 +62,9 @@ typedef cmp_func_t xfarray_cmp_fn;
 #define XFARRAY_ISORT_SHIFT		(4)
 #define XFARRAY_ISORT_NR		(1U << XFARRAY_ISORT_SHIFT)
 
+/* Evalulate this many points to find the qsort pivot. */
+#define XFARRAY_QSORT_PIVOT_NR		(9)
+
 struct xfarray_sortinfo {
 	struct xfarray		*array;
 
@@ -91,7 +94,6 @@ struct xfarray_sortinfo {
 	uint64_t		compares;
 	uint64_t		heapsorts;
 #endif
-
 	/*
 	 * Extra bytes are allocated beyond the end of the structure to store
 	 * quicksort information.  C does not permit multiple VLAs per struct,
@@ -114,11 +116,18 @@ struct xfarray_sortinfo {
 	 * 	xfarray_rec_t	scratch[ISORT_NR];
 	 *
 	 * Otherwise, we want to partition the records to partition the array.
-	 * We store the chosen pivot record here and use the xfarray scratchpad
-	 * to rearrange the array around the pivot:
-	 *
-	 * 	xfarray_rec_t	pivot;
+	 * We store the chosen pivot record at the start of the scratchpad area
+	 * and use the rest to sample some records to estimate the median.
+	 * The format of the qsort_pivot array enables us to use the kernel
+	 * heapsort function to place the median value in the middle.
 	 *
+	 * 	struct {
+	 * 		xfarray_rec_t	pivot;
+	 * 		struct {
+	 *			xfarray_rec_t	rec;  (rounded up to 8 bytes)
+	 * 			xfarray_idx_t	idx;
+	 *		} qsort_pivot[QSORT_PIVOT_NR];
+	 * 	};
 	 * }
 	 */
 };