From: Alexander Kozyrev Date: Fri, 20 Mar 2020 15:55:15 +0000 (+0000) Subject: mbuf: optimize memory loads during freeing X-Git-Url: http://git.droids-corp.org/?a=commitdiff_plain;h=759a6a221e8c31c3b7193cbd104753eaad01b89d;p=dpdk.git mbuf: optimize memory loads during freeing 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 // jump out to "if (m->next != NULL)" After the change the code became: movq 0x18(%rbx), %rax // load ol_flags test %r14, %rax // check if ol_flags equals to 0x60...0 jnz 0x9bea38 // jump in to "if (!RTE_MBUF_HAS_EXTBUF(m)" movq 0x48(%rbx), %rax // load the pool field jmp 0x9bea78 // 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 change versions. Plus, GCC favors the external 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 Acked-by: Viacheslav Ovsiienko Acked-by: Olivier Matz --- diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h index 34679e0920..f8e492e59d 100644 --- a/lib/librte_mbuf/rte_mbuf.h +++ b/lib/librte_mbuf/rte_mbuf.h @@ -1335,10 +1335,10 @@ rte_pktmbuf_prefree_seg(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_EXTBUF(m) && + RTE_MBUF_HAS_PINNED_EXTBUF(m) && + __rte_pktmbuf_pinned_extbuf_decref(m)) return NULL; } @@ -1352,10 +1352,10 @@ rte_pktmbuf_prefree_seg(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_EXTBUF(m) && + RTE_MBUF_HAS_PINNED_EXTBUF(m) && + __rte_pktmbuf_pinned_extbuf_decref(m)) return NULL; }