xarray: inline xas_descend to improve performance

The commit 63b1898 ("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

Link: https://lkml.kernel.org/r/20240416061628.3768901-1-leo.lilong@huawei.com
Signed-off-by: Long Li <leo.lilong@huawei.com>
Cc: Hou Tao <houtao1@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: yangerkun <yangerkun@huawei.com>
Cc: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit ba59180)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: Long Li

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);