netdev - Re: [Intel-wired-lan] [PATCH bpf-next V1] igc: enable and fix RX hash usage by netstack

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Message-ID: <6a5ded96-2425-ff9b-c1b1-eca1c103164c@molgen.mpg.de>
Date:   Tue, 14 Feb 2023 16:00:52 +0100
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, xdp-hints@...-project.net,
        martin.lau@...nel.org, daniel@...earbox.net,
        netdev@...r.kernel.org, ast@...nel.org,
        Stanislav Fomichev <sdf@...gle.com>,
        yoong.siang.song@...el.com, anthony.l.nguyen@...el.com,
        intel-wired-lan@...ts.osuosl.org
Subject: Re: [Intel-wired-lan] [PATCH bpf-next V1] igc: enable and fix RX hash
 usage by netstack

Dear Jesper,


Thank you very much for your patch.

Am 10.02.23 um 16:07 schrieb Jesper Dangaard Brouer:
> When function igc_rx_hash() was introduced in v4.20 via commit 0507ef8a0372
> ("igc: Add transmit and receive fastpath and interrupt handlers"), the
> hardware wasn't configured to provide RSS hash, thus it made sense to not
> enable net_device NETIF_F_RXHASH feature bit.
> 
> The NIC hardware was configured to enable RSS hash info in v5.2 via commit
> 2121c2712f82 ("igc: Add multiple receive queues control supporting"), but
> forgot to set the NETIF_F_RXHASH feature bit.
> 
> The original implementation of igc_rx_hash() didn't extract the associated
> pkt_hash_type, but statically set PKT_HASH_TYPE_L3. The largest portions of
> this patch are about extracting the RSS Type from the hardware and mapping
> this to enum pkt_hash_types. This were based on Foxville i225 software user

s/This were/This was/

> manual rev-1.3.1 and tested on Intel Ethernet Controller I225-LM (rev 03).
> 
> For UDP it's worth noting that RSS (type) hashing have been disabled both for
> IPv4 and IPv6 (see IGC_MRQC_RSS_FIELD_IPV4_UDP + IGC_MRQC_RSS_FIELD_IPV6_UDP)
> because hardware RSS doesn't handle fragmented pkts well when enabled (can
> cause out-of-order). This result in PKT_HASH_TYPE_L3 for UDP packets, and

result*s*

> hash value doesn't include UDP port numbers. Not being PKT_HASH_TYPE_L4, have
> the effect that netstack will do a software based hash calc calling into
> flow_dissect, but only when code calls skb_get_hash(), which doesn't
> necessary happen for local delivery.

Excuse my ignorance, but is that bug visible in practice by users 
(performance?) or is that fix needed for future work?

> Fixes: 2121c2712f82 ("igc: Add multiple receive queues control supporting")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>   drivers/net/ethernet/intel/igc/igc.h      |   52 +++++++++++++++++++++++++++++
>   drivers/net/ethernet/intel/igc/igc_main.c |   35 +++++++++++++++++---
>   2 files changed, 83 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/igc/igc.h b/drivers/net/ethernet/intel/igc/igc.h
> index df3e26c0cf01..a112eeb59525 100644
> --- a/drivers/net/ethernet/intel/igc/igc.h
> +++ b/drivers/net/ethernet/intel/igc/igc.h
> @@ -311,6 +311,58 @@ extern char igc_driver_name[];
>   #define IGC_MRQC_RSS_FIELD_IPV4_UDP	0x00400000
>   #define IGC_MRQC_RSS_FIELD_IPV6_UDP	0x00800000
>   
> +/* RX-desc Write-Back format RSS Type's */
> +enum igc_rss_type_num {
> +	IGC_RSS_TYPE_NO_HASH		= 0,
> +	IGC_RSS_TYPE_HASH_TCP_IPV4	= 1,
> +	IGC_RSS_TYPE_HASH_IPV4		= 2,
> +	IGC_RSS_TYPE_HASH_TCP_IPV6	= 3,
> +	IGC_RSS_TYPE_HASH_IPV6_EX	= 4,
> +	IGC_RSS_TYPE_HASH_IPV6		= 5,
> +	IGC_RSS_TYPE_HASH_TCP_IPV6_EX	= 6,
> +	IGC_RSS_TYPE_HASH_UDP_IPV4	= 7,
> +	IGC_RSS_TYPE_HASH_UDP_IPV6	= 8,
> +	IGC_RSS_TYPE_HASH_UDP_IPV6_EX	= 9,
> +	IGC_RSS_TYPE_MAX		= 10,
> +};
> +#define IGC_RSS_TYPE_MAX_TABLE		16
> +#define IGC_RSS_TYPE_MASK		0xF
> +
> +/* igc_rss_type - Rx descriptor RSS type field */
> +static inline u8 igc_rss_type(union igc_adv_rx_desc *rx_desc)
> +{
> +	/* RSS Type 4-bit number: 0-9 (above 9 is reserved) */
> +	return rx_desc->wb.lower.lo_dword.hs_rss.pkt_info & IGC_RSS_TYPE_MASK;
> +}

Is it necessary to specficy the length of the return value, or could it 
be `unsigned int`. Using “native” types is normally more performant [1]. 
`scripts/bloat-o-meter` might help to verify that.

[…]

>   static inline void igc_rx_hash(struct igc_ring *ring,
>   			       union igc_adv_rx_desc *rx_desc,
>   			       struct sk_buff *skb)
>   {
> -	if (ring->netdev->features & NETIF_F_RXHASH)
> -		skb_set_hash(skb,
> -			     le32_to_cpu(rx_desc->wb.lower.hi_dword.rss),
> -			     PKT_HASH_TYPE_L3);
> +	if (ring->netdev->features & NETIF_F_RXHASH) {
> +		u32 rss_hash = le32_to_cpu(rx_desc->wb.lower.hi_dword.rss);
> +		u8  rss_type = igc_rss_type(rx_desc);

Amongst others, also here.

> +		enum pkt_hash_types hash_type;
> +
> +		hash_type = igc_rss_type_table[rss_type].hash_type;
> +		skb_set_hash(skb, rss_hash, hash_type);
> +	}
>   }
>   
>   static void igc_rx_vlan(struct igc_ring *rx_ring,
> @@ -6501,6 +6527,7 @@ static int igc_probe(struct pci_dev *pdev,
>   	netdev->features |= NETIF_F_TSO;
>   	netdev->features |= NETIF_F_TSO6;
>   	netdev->features |= NETIF_F_TSO_ECN;
> +	netdev->features |= NETIF_F_RXHASH;
>   	netdev->features |= NETIF_F_RXCSUM;
>   	netdev->features |= NETIF_F_HW_CSUM;
>   	netdev->features |= NETIF_F_SCTP_CRC;


Kind regards,

Paul


[1]: https://notabs.org/coding/smallIntsBigPenalty.htm