netdev - Re: [Intel-wired-lan] [PATCH net v2 6/6] igc: Fix inserting of empty frame for launchtime

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Date: Thu, 6 Jul 2023 14:36:26 +0300
From: "naamax.meir" <naamax.meir@...ux.intel.com>
To: Florian Kauer <florian.kauer@...utronix.de>,
 Jesse Brandeburg <jesse.brandeburg@...el.com>,
 Tony Nguyen <anthony.l.nguyen@...el.com>,
 Vinicius Costa Gomes <vinicius.gomes@...el.com>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Tan Tee Min <tee.min.tan@...ux.intel.com>,
 Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@...el.com>,
 Aravindhan Gunasekaran <aravindhan.gunasekaran@...el.com>,
 Malli C <mallikarjuna.chilakala@...el.com>
Cc: netdev@...r.kernel.org, kurt@...utronix.de,
 intel-wired-lan@...ts.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [Intel-wired-lan] [PATCH net v2 6/6] igc: Fix inserting of empty
 frame for launchtime

On 6/19/2023 13:08, Florian Kauer wrote:
> The insertion of an empty frame was introduced with
> commit db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag bit")
> in order to ensure that the current cycle has at least one packet if
> there is some packet to be scheduled for the next cycle.
> 
> However, the current implementation does not properly check if
> a packet is already scheduled for the current cycle. Currently,
> an empty packet is always inserted if and only if
> txtime >= end_of_cycle && txtime > last_tx_cycle
> but since last_tx_cycle is always either the end of the current
> cycle (end_of_cycle) or the end of a previous cycle, the
> second part (txtime > last_tx_cycle) is always true unless
> txtime == last_tx_cycle.
> 
> What actually needs to be checked here is if the last_tx_cycle
> was already written within the current cycle, so an empty frame
> should only be inserted if and only if
> txtime >= end_of_cycle && end_of_cycle > last_tx_cycle.
> 
> This patch does not only avoid an unnecessary insertion, but it
> can actually be harmful to insert an empty packet if packets
> are already scheduled in the current cycle, because it can lead
> to a situation where the empty packet is actually processed
> as the first packet in the upcoming cycle shifting the packet
> with the first_flag even one cycle into the future, finally leading
> to a TX hang.
> 
> The TX hang can be reproduced on a i225 with:
> 
>      sudo tc qdisc replace dev enp1s0 parent root handle 100 taprio \
> 	    num_tc 1 \
> 	    map 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 \
> 	    queues 1@0 \
> 	    base-time 0 \
> 	    sched-entry S 01 300000 \
> 	    flags 0x1 \
> 	    txtime-delay 500000 \
> 	    clockid CLOCK_TAI
>      sudo tc qdisc replace dev enp1s0 parent 100:1 etf \
> 	    clockid CLOCK_TAI \
> 	    delta 500000 \
> 	    offload \
> 	    skip_sock_check
> 
> and traffic generator
> 
>      sudo trafgen -i traffic.cfg -o enp1s0 --cpp -n0 -q -t1400ns
> 
> with traffic.cfg
> 
>      #define ETH_P_IP        0x0800
> 
>      {
>        /* Ethernet Header */
>        0x30, 0x1f, 0x9a, 0xd0, 0xf0, 0x0e,  # MAC Dest - adapt as needed
>        0x24, 0x5e, 0xbe, 0x57, 0x2e, 0x36,  # MAC Src  - adapt as needed
>        const16(ETH_P_IP),
> 
>        /* IPv4 Header */
>        0b01000101, 0,   # IPv4 version, IHL, TOS
>        const16(1028),   # IPv4 total length (UDP length + 20 bytes (IP header))
>        const16(2),      # IPv4 ident
>        0b01000000, 0,   # IPv4 flags, fragmentation off
>        64,              # IPv4 TTL
>        17,              # Protocol UDP
>        csumip(14, 33),  # IPv4 checksum
> 
>        /* UDP Header */
>        10,  0, 48, 1,   # IP Src - adapt as needed
>        10,  0, 48, 10,  # IP Dest - adapt as needed
>        const16(5555),   # UDP Src Port
>        const16(6666),   # UDP Dest Port
>        const16(1008),   # UDP length (UDP header 8 bytes + payload length)
>        csumudp(14, 34), # UDP checksum
> 
>        /* Payload */
>        fill('W', 1000),
>      }
> 
> and the observed message with that is for example
> 
>   igc 0000:01:00.0 enp1s0: Detected Tx Unit Hang
>     Tx Queue             <0>
>     TDH                  <32>
>     TDT                  <3c>
>     next_to_use          <3c>
>     next_to_clean        <32>
>   buffer_info[next_to_clean]
>     time_stamp           <ffff26a8>
>     next_to_watch        <00000000632a1828>
>     jiffies              <ffff27f8>
>     desc.status          <1048000>
> 
> Fixes: db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag bit")
> Signed-off-by: Florian Kauer <florian.kauer@...utronix.de>
> Reviewed-by: Kurt Kanzenbach <kurt@...utronix.de>
> ---
>   drivers/net/ethernet/intel/igc/igc_main.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)

Tested-by: Naama Meir <naamax.meir@...ux.intel.com>