[PATCH v2] xarray: inline xas_descend to improve performance

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The commit 63b1898fffcd ("XArray: Disallow sibling entries of nodes")
modified the xas_descend function in such a way that it was no longer
being compiled as an inline function, because it increased the size of
xas_descend(), and the compiler no longer optimizes it as inline. This
had a negative impact on performance, xas_descend is called frequently
to traverse downwards in the xarray tree, making it a hot function.

Inlining xas_descend has been shown to significantly improve performance
by approximately 4.95% in the iozone write test.

  Machine: Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz
  #iozone i 0 -i 1 -s 64g -r 16m -f /test/tmptest

Before this patch:

       kB    reclen    write   rewrite     read    reread
 67108864     16384  2230080   3637689  6315197   5496027

After this patch:

       kB    reclen    write   rewrite     read    reread
 67108864     16384  2340360   3666175  6272401   5460782

Percentage change:
                       4.95%     0.78%   -0.68%    -0.64%

This patch introduces inlining to the xas_descend function. While this
change increases the size of lib/xarray.o, the performance gains in
critical workloads make this an acceptable trade-off.

Size comparison before and after patch:
.text		.data		.bss		file
0x3502		    0		   0		lib/xarray.o.before
0x3602		    0		   0		lib/xarray.o.after

Signed-off-by: Long Li <leo.lilong@xxxxxxxxxx>
 - Using '__always_inline' instead of 'inline' guarantees that xas_descend
 will definitely be inlined. 
 lib/xarray.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/xarray.c b/lib/xarray.c
index fbf1d1dd83bc..84faa433cd24 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -200,7 +200,8 @@ static void *xas_start(struct xa_state *xas)
 	return entry;
-static void *xas_descend(struct xa_state *xas, struct xa_node *node)
+static __always_inline void *xas_descend(struct xa_state *xas,
+					struct xa_node *node)
 	unsigned int offset = get_offset(xas->xa_index, node);
 	void *entry = xa_entry(xas->xa, node, offset);

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