[dpdk-dev] [PATCH] mbuf: optimize memory loads during mbuf freeing

[Alexander Kozyrev <akozyrev@mellanox.com>]

Introduction of pinned external buffers doubled memory loads in the
rte_pktmbuf_prefree_seg() function. Analysis of the generated assembly
code shows unnecessary load of the pool field of the rte_mbuf structure.
Here is the snippet of the assembly for "if (!RTE_MBUF_DIRECT(m))":
Before the change the code was:
	movq  0x18(%rbx), %rax // load the ol_flags field
	test %r13, %rax	       // check if ol_flags equals to 0x60...0
	jz 0x9a8718 <Block 2>  // jump out to "if (m->next != NULL)"
After the change the code becomed:
	movq  0x18(%rbx), %rax // load ol_flags
	test %r14, %rax	       // check if ol_flags equals to 0x60...0
	jnz 0x9bea38 <Block 2> // jump in to "if (!RTE_MBUF_HAS_EXTBUF(m)"
	movq  0x48(%rbx), %rax // load the pool field
	jmp 0x9bea78 <Block 7> // jump out to "if (m->next != NULL)"
Look like this absolutely unneeded memory load of the pool field is an
optimization for the external buffer case in GCC (4.8.5), since Clang
generates the same assembly for both before and after the chenge versions.
Plus, GCC favors the extrnal buffer case over the simple case.
This assembly code layout causes the performance degradation because the
rte_pktmbuf_prefree_seg() function is a part of a very hot path.
Workaround this compilation issue by moving the check for pinned buffer
apart from the check for external buffer and restore the initial code
flow that favors the direct mbuf case over the external one.

Fixes: 6ef1107ad4c6 ("mbuf: detach mbuf with pinned external buffer")
Cc: stable@dpdk.org

Signed-off-by: Alexander Kozyrev <akozyrev@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
---
 lib/librte_mbuf/rte_mbuf.h | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 34679e0..ab9d3f5 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -1335,10 +1335,9 @@ static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
 	if (likely(rte_mbuf_refcnt_read(m) == 1)) {
 
 		if (!RTE_MBUF_DIRECT(m)) {
-			if (!RTE_MBUF_HAS_EXTBUF(m) ||
-			    !RTE_MBUF_HAS_PINNED_EXTBUF(m))
-				rte_pktmbuf_detach(m);
-			else if (__rte_pktmbuf_pinned_extbuf_decref(m))
+			rte_pktmbuf_detach(m);
+			if (RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
+			    __rte_pktmbuf_pinned_extbuf_decref(m))
 				return NULL;
 		}
 
@@ -1352,10 +1351,9 @@ static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
 	} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
 
 		if (!RTE_MBUF_DIRECT(m)) {
-			if (!RTE_MBUF_HAS_EXTBUF(m) ||
-			    !RTE_MBUF_HAS_PINNED_EXTBUF(m))
-				rte_pktmbuf_detach(m);
-			else if (__rte_pktmbuf_pinned_extbuf_decref(m))
+			rte_pktmbuf_detach(m);
+			if (RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
+			    __rte_pktmbuf_pinned_extbuf_decref(m))
 				return NULL;
 		}
 
-- 
1.8.3.1