svn commit: r311877 - head/sys/dev/sfxge

[Andrew Rybchenko]

Author: arybchik
Date: Tue Jan 10 16:25:39 2017
New Revision: 311877
URL: https://svnweb.freebsd.org/changeset/base/311877

Log:
  sfxge(4): avoid unnecessary mbuf data prefetch
  
  Unnecessary prefetch just loads HW prefetcher and displaces other
  cache entries (which could be really useful).
  
  If we parse mbuf for TSO early and use firmware-assisted TSO, we do not
  expect mbuf data access when we compose firmware-assisted TSO (v1 or v2)
  option descriptors.  If packet header needs to be linearized or finally
  FATSO cannot be used because of, for example, too big header, we do not
  care about a bit more performance degradation because of prefetch
  absence (it is better to optimize more common case).
  
  Reviewed by:    gnn
  Sponsored by:   Solarflare Communications, Inc.
  MFC after:      2 days
  Differential Revision:  https://reviews.freebsd.org/D9120

Modified:
  head/sys/dev/sfxge/sfxge_tx.c

Modified: head/sys/dev/sfxge/sfxge_tx.c
==============================================================================
--- head/sys/dev/sfxge/sfxge_tx.c	Tue Jan 10 14:40:30 2017	(r311876)
+++ head/sys/dev/sfxge/sfxge_tx.c	Tue Jan 10 16:25:39 2017	(r311877)
@@ -363,8 +363,22 @@ static int sfxge_tx_queue_mbuf(struct sf
 
 	KASSERT(!txq->blocked, ("txq->blocked"));
 
+#if SFXGE_TX_PARSE_EARLY
+	/*
+	 * If software TSO is used, we still need to copy packet header,
+	 * even if we have already parsed it early before enqueue.
+	 */
+	if ((mbuf->m_pkthdr.csum_flags & CSUM_TSO) &&
+	    (txq->tso_fw_assisted == 0))
+		prefetch_read_many(mbuf->m_data);
+#else
+	/*
+	 * Prefetch packet header since we need to parse it and extract
+	 * IP ID, TCP sequence number and flags.
+	 */
 	if (mbuf->m_pkthdr.csum_flags & CSUM_TSO)
 		prefetch_read_many(mbuf->m_data);
+#endif
 
 	if (__predict_false(txq->init_state != SFXGE_TXQ_STARTED)) {
 		rc = EINTR;