[RFC PATCH v2 1/3] cache: conditionally align cache groups

[Zecheng Li]

Introduces a pair of macros, `__cacheline_group_begin_aligned_cond` and
`__cacheline_group_end_aligned_cond`, to provide conditional cacheline
alignment for cache groups. The alignment behavior is as follows:

If the `COND` parameter is equal to `SMP_CACHE_BYTES`, the cache group
will be aligned to `SMP_CACHE_BYTES`.

If `COND` is not equal to `SMP_CACHE_BYTES`, no specific additional
cacheline alignment is enforced by using `__aligned(1)`.

This mechanism allows for more precise control over cacheline alignment,
ensuring that layout optimizations intended for one cache architecture
do not inadvertently degrade efficiency or introduce holes on systems
with different cache line sizes.

Signed-off-by: Zecheng Li <zecheng@xxxxxxxxxx>
---
 include/linux/cache.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/include/linux/cache.h b/include/linux/cache.h
index e69768f50d53..8b5dadf1c487 100644
--- a/include/linux/cache.h
+++ b/include/linux/cache.h
@@ -147,6 +147,34 @@
 	struct { } __cacheline_group_pad__##GROUP		\
 	__aligned((__VA_ARGS__ + 0) ? : SMP_CACHE_BYTES)
 
+/**
+ * __cacheline_group_begin_aligned_cond - conditionally align a cache group
+ * @GROUP: name of the group
+ * @COND: a size; if it equals SMP_CACHE_BYTES, the group will be aligned
+ * to SMP_CACHE_BYTES. Otherwise, no specific cacheline alignment
+ * is enforced.
+ *
+ */
+#define __cacheline_group_begin_aligned_cond(GROUP, COND)	\
+	__cacheline_group_begin(GROUP)				\
+	__aligned(((COND) == SMP_CACHE_BYTES) ? SMP_CACHE_BYTES : 1)
+
+/**
+ * __cacheline_group_end_aligned_cond - declare a conditionally aligned group end
+ * @GROUP: name of the group
+ * @COND: condition (size); if it equals SMP_CACHE_BYTES, padding will
+ * be aligned to SMP_CACHE_BYTES. Otherwise, no alignment.
+ *
+ * This complements __cacheline_group_begin_aligned_cond.
+ * The end marker itself is aligned to sizeof(long).
+ * The final padding to avoid the next field falling into this cacheline
+ * is applied conditionally based on COND.
+ */
+#define __cacheline_group_end_aligned_cond(GROUP, COND)                 \
+        __cacheline_group_end(GROUP) __aligned(sizeof(long));           \
+        struct { } __cacheline_group_pad__##GROUP                       \
+        __aligned(((COND) == SMP_CACHE_BYTES) ? SMP_CACHE_BYTES : 1)
+
 #ifndef CACHELINE_ASSERT_GROUP_MEMBER
 #define CACHELINE_ASSERT_GROUP_MEMBER(TYPE, GROUP, MEMBER) \
 	BUILD_BUG_ON(!(offsetof(TYPE, MEMBER) >= \
-- 
2.49.0