Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for all NICs but 82598

["Zhang, Helin" <helin.zhang@intel.com>]

Hi Vlad

I think this could possibly be the root cause of your TX hang issue. Please try to limit the number to 8 or less, and then see if the issue will still be there or not?
It does not have any check for the number of descriptors to be used for a single packet, and it relies on the users to give correct mbuf chains.

We may need a check of this somewhere. Of cause the point you indicated we also need to carefully investigate or fix.

Regards,
Helin

From: Vladislav Zolotarov [mailto:vladz@cloudius-systems.com]
Sent: Tuesday, August 25, 2015 11:34 AM
To: Zhang, Helin
Cc: Lu, Wenzhuo; dev@dpdk.org
Subject: RE: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for all NICs but 82598


On Aug 25, 2015 21:14, "Zhang, Helin" <helin.zhang@intel.com<mailto:helin.zhang@intel.com>> wrote:
>
> Hi Vlad
>
>
>
> In addition, I’d double check with you what’s the maximum number of descriptors would be used for a single packet transmitting?
>
> Datasheet said that it supports up to 8. I am wondering if more than 8 were used in your case?

If memory serves me well the maximum number of data descriptors per single xmit packet is 40 minus 2 minus WTHRESH. Since WTHRESH in DPDK is always zero it gives us 38 segments. We limit them by 33.

>
> Thank you very much!
>
>
>
> Regards,
>
> Helin
>
>
>
> From: Zhang, Helin
> Sent: Wednesday, August 19, 2015 10:29 AM
> To: Vladislav Zolotarov
> Cc: Lu, Wenzhuo; dev@dpdk.org<mailto:dev@dpdk.org>
>
> Subject: RE: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for all NICs but 82598
>
>
>
> Hi Vlad
>
>
>
> Thank you very much for the patches! Give me a few more time to double check with more guys, and possibly hardware experts.
>
>
>
> Regards,
>
> Helin
>
>
>
> From: Vladislav Zolotarov [mailto:vladz@cloudius-systems.com<mailto:vladz@cloudius-systems.com>]
> Sent: Tuesday, August 18, 2015 9:56 PM
> To: Lu, Wenzhuo
> Cc: dev@dpdk.org<mailto:dev@dpdk.org>; Zhang, Helin
> Subject: RE: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for all NICs but 82598
>
>
>
>
> On Aug 19, 2015 03:42, "Lu, Wenzhuo" <wenzhuo.lu@intel.com<mailto:wenzhuo.lu@intel.com>> wrote:
> >
> > Hi Helin,
> >
> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces@dpdk.org<mailto:dev-bounces@dpdk.org>] On Behalf Of Vlad Zolotarov
> > > Sent: Friday, August 14, 2015 1:38 PM
> > > To: Zhang, Helin; dev@dpdk.org<mailto:dev@dpdk.org>
> > > Subject: Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for
> > > all NICs but 82598
> > >
> > >
> > >
> > > On 08/13/15 23:28, Zhang, Helin wrote:
> > > > Hi Vlad
> > > >
> > > > I don't think the changes are needed. It says in datasheet that the RS
> > > > bit should be set on the last descriptor of every packet, ONLY WHEN
> > > TXDCTL.WTHRESH equals to ZERO.
> > >
> > > Of course it's needed! See below.
> > > Exactly the same spec a few lines above the place u've just quoted states:
> > >
> > > "Software should not set the RS bit when TXDCTL.WTHRESH is greater than
> > > zero."
> > >
> > > And since all three (3) ixgbe xmit callbacks are utilizing RS bit notation ixgbe PMD
> > > is actually not supporting any value of WTHRESH different from zero.
> > I think Vlad is right. We need to fix this issue. Any suggestion? If not, I'd like to ack this patch.
>
> Pls., note that there is a v2 of this patch on the list. I forgot to patch ixgbevf_dev_info_get() in v1.
>
> >
> > >
> > > >
> > > > Regards,
> > > > Helin
> > > >
> > > >> -----Original Message-----
> > > >> From: Vlad Zolotarov [mailto:vladz@cloudius-systems.com<mailto:vladz@cloudius-systems.com>]
> > > >> Sent: Thursday, August 13, 2015 11:07 AM
> > > >> To: dev@dpdk.org<mailto:dev@dpdk.org>
> > > >> Cc: Zhang, Helin; Ananyev, Konstantin; avi@cloudius-systems.com<mailto:avi@cloudius-systems.com>; Vlad
> > > >> Zolotarov
> > > >> Subject: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for
> > > all
> > > >> NICs but 82598
> > > >>
> > > >> According to 82599 and x540 HW specifications RS bit *must* be set in the
> > > last
> > > >> descriptor of *every* packet.
> > > > There is a condition that if TXDCTL.WTHRESH equal to zero.
> > >
> > > Right and ixgbe PMD requires this condition to be fulfilled in order to
> > > function. See above.
> > >
> > > >
> > > >> This patch fixes the Tx hang we were constantly hitting with a seastar-based
> > > >> application on x540 NIC.
> > > > Could you help to share with us how to reproduce the tx hang issue, with using
> > > > typical DPDK examples?
> > >
> > > Sorry. I'm not very familiar with the typical DPDK examples to help u
> > > here. However this is quite irrelevant since without this this patch
> > > ixgbe PMD obviously abuses the HW spec as has been explained above.
> > >
> > > We saw the issue when u stressed the xmit path with a lot of highly
> > > fragmented TCP frames (packets with up to 33 fragments with non-headers
> > > fragments as small as 4 bytes) with all offload features enabled.
> > >
> > > Thanks,
> > > vlad
> > > >
> > > >> Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com<mailto:vladz@cloudius-systems.com>>
> > > >> ---
> > > >>   drivers/net/ixgbe/ixgbe_ethdev.c |  9 +++++++++
> > > >>   drivers/net/ixgbe/ixgbe_rxtx.c   | 23 ++++++++++++++++++++++-
> > > >>   2 files changed, 31 insertions(+), 1 deletion(-)
> > > >>
> > > >> diff --git a/drivers/net/ixgbe/ixgbe_ethdev.c
> > > >> b/drivers/net/ixgbe/ixgbe_ethdev.c
> > > >> index b8ee1e9..6714fd9 100644
> > > >> --- a/drivers/net/ixgbe/ixgbe_ethdev.c
> > > >> +++ b/drivers/net/ixgbe/ixgbe_ethdev.c
> > > >> @@ -2414,6 +2414,15 @@ ixgbe_dev_info_get(struct rte_eth_dev *dev,
> > > struct
> > > >> rte_eth_dev_info *dev_info)
> > > >>            .txq_flags = ETH_TXQ_FLAGS_NOMULTSEGS |
> > > >>                            ETH_TXQ_FLAGS_NOOFFLOADS,
> > > >>    };
> > > >> +
> > > >> +  /*
> > > >> +   * According to 82599 and x540 specifications RS bit *must* be set on
> > > the
> > > >> +   * last descriptor of *every* packet. Therefore we will not allow the
> > > >> +   * tx_rs_thresh above 1 for all NICs newer than 82598.
> > > >> +   */
> > > >> +  if (hw->mac.type > ixgbe_mac_82598EB)
> > > >> +          dev_info->default_txconf.tx_rs_thresh = 1;
> > > >> +
> > > >>    dev_info->hash_key_size = IXGBE_HKEY_MAX_INDEX * sizeof(uint32_t);
> > > >>    dev_info->reta_size = ETH_RSS_RETA_SIZE_128;
> > > >>    dev_info->flow_type_rss_offloads = IXGBE_RSS_OFFLOAD_ALL; diff --
> > > git
> > > >> a/drivers/net/ixgbe/ixgbe_rxtx.c b/drivers/net/ixgbe/ixgbe_rxtx.c index
> > > >> 91023b9..8dbdffc 100644
> > > >> --- a/drivers/net/ixgbe/ixgbe_rxtx.c
> > > >> +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
> > > >> @@ -2085,11 +2085,19 @@ ixgbe_dev_tx_queue_setup(struct rte_eth_dev
> > > >> *dev,
> > > >>    struct ixgbe_tx_queue *txq;
> > > >>    struct ixgbe_hw     *hw;
> > > >>    uint16_t tx_rs_thresh, tx_free_thresh;
> > > >> +  bool rs_deferring_allowed;
> > > >>
> > > >>    PMD_INIT_FUNC_TRACE();
> > > >>    hw = IXGBE_DEV_PRIVATE_TO_HW(dev->data->dev_private);
> > > >>
> > > >>    /*
> > > >> +   * According to 82599 and x540 specifications RS bit *must* be set on
> > > the
> > > >> +   * last descriptor of *every* packet. Therefore we will not allow the
> > > >> +   * tx_rs_thresh above 1 for all NICs newer than 82598.
> > > >> +   */
> > > >> +  rs_deferring_allowed = (hw->mac.type <= ixgbe_mac_82598EB);
> > > >> +
> > > >> +  /*
> > > >>     * Validate number of transmit descriptors.
> > > >>     * It must not exceed hardware maximum, and must be multiple
> > > >>     * of IXGBE_ALIGN.
> > > >> @@ -2110,6 +2118,8 @@ ixgbe_dev_tx_queue_setup(struct rte_eth_dev
> > > *dev,
> > > >>     * to transmit a packet is greater than the number of free TX
> > > >>     * descriptors.
> > > >>     * The following constraints must be satisfied:
> > > >> +   *  tx_rs_thresh must be less than 2 for NICs for which RS deferring is
> > > >> +   *  forbidden (all but 82598).
> > > >>     *  tx_rs_thresh must be greater than 0.
> > > >>     *  tx_rs_thresh must be less than the size of the ring minus 2.
> > > >>     *  tx_rs_thresh must be less than or equal to tx_free_thresh.
> > > >> @@ -2121,9 +2131,20 @@ ixgbe_dev_tx_queue_setup(struct rte_eth_dev
> > > *dev,
> > > >>     * When set to zero use default values.
> > > >>     */
> > > >>    tx_rs_thresh = (uint16_t)((tx_conf->tx_rs_thresh) ?
> > > >> -                  tx_conf->tx_rs_thresh : DEFAULT_TX_RS_THRESH);
> > > >> +                  tx_conf->tx_rs_thresh :
> > > >> +                  (rs_deferring_allowed ? DEFAULT_TX_RS_THRESH : 1));
> > > >>    tx_free_thresh = (uint16_t)((tx_conf->tx_free_thresh) ?
> > > >>                    tx_conf->tx_free_thresh : DEFAULT_TX_FREE_THRESH);
> > > >> +
> > > >> +  if (!rs_deferring_allowed && tx_rs_thresh > 1) {
> > > >> +          PMD_INIT_LOG(ERR, "tx_rs_thresh must be less than 2 since RS
> > > "
> > > >> +                            "must be set for every packet for this HW. "
> > > >> +                            "(tx_rs_thresh=%u port=%d queue=%d)",
> > > >> +                       (unsigned int)tx_rs_thresh,
> > > >> +                       (int)dev->data->port_id, (int)queue_idx);
> > > >> +          return -(EINVAL);
> > > >> +  }
> > > >> +
> > > >>    if (tx_rs_thresh >= (nb_desc - 2)) {
> > > >>            PMD_INIT_LOG(ERR, "tx_rs_thresh must be less than the
> > > number "
> > > >>                         "of TX descriptors minus 2. (tx_rs_thresh=%u "
> > > >> --
> > > >> 2.1.0
> >