[PATCH] clarify purpose of empty cache lines

[Morten Brørup]

This patch introduces the generic RTE_CACHE_GUARD macro into the EAL, and
replaces vaguely described empty cache lines in the rte_ring structure
with this macro.

Although the implementation of the rte_ring structure assumes that the
hardware speculatively prefetches 1 cache line, this number can be changed
at build time by modifying RTE_CACHE_GUARD_LINES in rte_config.h.

The background and the RFC was discussed in this thread:
http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35D87B39@smartserver.smartshare.dk/

Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
---
 config/rte_config.h          |  1 +
 lib/eal/include/rte_common.h | 13 +++++++++++++
 lib/ring/rte_ring_core.h     |  6 +++---
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/config/rte_config.h b/config/rte_config.h
index 400e44e3cf..cfdf787724 100644
--- a/config/rte_config.h
+++ b/config/rte_config.h
@@ -37,6 +37,7 @@
 #define RTE_MAX_TAILQ 32
 #define RTE_LOG_DP_LEVEL RTE_LOG_INFO
 #define RTE_MAX_VFIO_CONTAINERS 64
+#define RTE_CACHE_GUARD_LINES 1
 
 /* bsd module defines */
 #define RTE_CONTIGMEM_MAX_NUM_BUFS 64
diff --git a/lib/eal/include/rte_common.h b/lib/eal/include/rte_common.h
index 771c70f2c8..daf1866a32 100644
--- a/lib/eal/include/rte_common.h
+++ b/lib/eal/include/rte_common.h
@@ -527,6 +527,19 @@ rte_is_aligned(const void * const __rte_restrict ptr, const unsigned int align)
 /** Force minimum cache line alignment. */
 #define __rte_cache_min_aligned __rte_aligned(RTE_CACHE_LINE_MIN_SIZE)
 
+#define _RTE_CACHE_GUARD_HELPER2(unique) \
+		char cache_guard_ ## unique[RTE_CACHE_LINE_SIZE * RTE_CACHE_GUARD_LINES] \
+		__rte_cache_aligned
+#define _RTE_CACHE_GUARD_HELPER1(unique) _RTE_CACHE_GUARD_HELPER2(unique)
+/**
+ * Empty cache lines, to guard against false sharing-like effects
+ * on systems with a next-N-lines hardware prefetcher.
+ *
+ * Use as spacing between data accessed by different lcores,
+ * to prevent cache thrashing on hardware with speculative prefetching.
+ */
+#define RTE_CACHE_GUARD _RTE_CACHE_GUARD_HELPER1(__COUNTER__)
+
 /*********** PA/IOVA type definitions ********/
 
 /** Physical address */
diff --git a/lib/ring/rte_ring_core.h b/lib/ring/rte_ring_core.h
index d1e59bf9ad..327fdcf28f 100644
--- a/lib/ring/rte_ring_core.h
+++ b/lib/ring/rte_ring_core.h
@@ -126,7 +126,7 @@ struct rte_ring {
 	uint32_t mask;           /**< Mask (size-1) of ring. */
 	uint32_t capacity;       /**< Usable size of ring */
 
-	char pad0 __rte_cache_aligned; /**< empty cache line */
+	RTE_CACHE_GUARD;
 
 	/** Ring producer status. */
 	union {
@@ -135,7 +135,7 @@ struct rte_ring {
 		struct rte_ring_rts_headtail rts_prod;
 	}  __rte_cache_aligned;
 
-	char pad1 __rte_cache_aligned; /**< empty cache line */
+	RTE_CACHE_GUARD;
 
 	/** Ring consumer status. */
 	union {
@@ -144,7 +144,7 @@ struct rte_ring {
 		struct rte_ring_rts_headtail rts_cons;
 	}  __rte_cache_aligned;
 
-	char pad2 __rte_cache_aligned; /**< empty cache line */
+	RTE_CACHE_GUARD;
 };
 
 #define RING_F_SP_ENQ 0x0001 /**< The default enqueue is "single-producer". */
-- 
2.17.1