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sfxge(4): avoid unnecessary mbuf data prefetch

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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
This commit is contained in:
Andrew Rybchenko 2017-01-10 16:25:39 +00:00
parent 4a6768e64c
commit e3ef7bb216
1 changed files with 14 additions and 0 deletions
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14
sys/dev/sfxge/sfxge_tx.c
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@ -363,8 +363,22 @@ static int sfxge_tx_queue_mbuf(struct sfxge_txq *txq, struct mbuf *mbuf)
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;