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Date: Thu, 16 Feb 2023 17:46:53 +0100
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
To: Alexander Lobakin <alexandr.lobakin@...el.com>
Cc: brouer@...hat.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
Stanislav Fomichev <sdf@...gle.com>, martin.lau@...nel.org,
ast@...nel.org, daniel@...earbox.net, yoong.siang.song@...el.com,
anthony.l.nguyen@...el.com, intel-wired-lan@...ts.osuosl.org,
xdp-hints@...-project.net
Subject: Re: [PATCH bpf-next V1] igc: enable and fix RX hash usage by netstack
On 14/02/2023 14.21, Alexander Lobakin wrote:
> From: Jesper Dangaard Brouer <brouer@...hat.com>
> Date: Fri, 10 Feb 2023 16:07:59 +0100
>
>> 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.
>
> [...]
>
>> @@ -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
>
> GENMASK()?
>
hmm... GENMASK(3,0) looks more confusing to me. The mask we need here is
so simple that I prefer not to complicate this with GENMASK.
>> +
>> +/* igc_rss_type - Rx descriptor RSS type field */
>> +static inline u8 igc_rss_type(union igc_adv_rx_desc *rx_desc)
>
> Why use types shorter than u32 on the stack?
Changing to u32 in V2
> Why this union is not const here, since there are no modifications?
Sure
>> +{
>> + /* 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;
>
> The most important I wanted to mention: doesn't this function make the
> CPU read the uncached field again, while you could just read it once
> onto the stack and then extract all such data from there?
I really don't think this is an issues here. The igc_adv_rx_desc is only
16 bytes and it should be hot in CPU cache by now.
To avoid the movzx I have changed this to do a u32 read instead.
>> +}
>> +
>> +/* Packet header type identified by hardware (when BIT(11) is zero).
>> + * Even when UDP ports are not part of RSS hash HW still parse and mark UDP bits
>> + */
>> +enum igc_pkt_type_bits {
>> + IGC_PKT_TYPE_HDR_IPV4 = BIT(0),
>> + IGC_PKT_TYPE_HDR_IPV4_WITH_OPT= BIT(1), /* IPv4 Hdr includes IP options */
>> + IGC_PKT_TYPE_HDR_IPV6 = BIT(2),
>> + IGC_PKT_TYPE_HDR_IPV6_WITH_EXT= BIT(3), /* IPv6 Hdr includes extensions */
>> + IGC_PKT_TYPE_HDR_L4_TCP = BIT(4),
>> + IGC_PKT_TYPE_HDR_L4_UDP = BIT(5),
>> + IGC_PKT_TYPE_HDR_L4_SCTP= BIT(6),
>> + IGC_PKT_TYPE_HDR_NFS = BIT(7),
>> + /* Above only valid when BIT(11) is zero */
>> + IGC_PKT_TYPE_L2 = BIT(11),
>> + IGC_PKT_TYPE_VLAN = BIT(12),
>> + IGC_PKT_TYPE_MASK = 0x1FFF, /* 13-bits */
>
> Also GENMASK().
GENMASK would make more sense here.
>> +};
>> +
>> +/* igc_pkt_type - Rx descriptor Packet type field */
>> +static inline u16 igc_pkt_type(union igc_adv_rx_desc *rx_desc)
>
> Also short types and consts.
>
Fixed in V2
>> +{
>> + u32 data = le32_to_cpu(rx_desc->wb.lower.lo_dword.data);
>> + /* Packet type is 13-bits - as bits (16:4) in lower.lo_dword*/
>> + u16 pkt_type = (data >> 4) & IGC_PKT_TYPE_MASK;
>
> Perfect candidate for FIELD_GET(). No, even for le32_get_bits().
I adjusted this, but I could not find a central define for FIELD_GET
(but many drivers open code this).
> Also my note above about excessive expensive reads.
>
>> +
>> + return pkt_type;
>> +}
>> +
>> /* Interrupt defines */
>> #define IGC_START_ITR 648 /* ~6000 ints/sec */
>> #define IGC_4K_ITR 980
>> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
>> index 8b572cd2c350..42a072509d2a 100644
>> --- a/drivers/net/ethernet/intel/igc/igc_main.c
>> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
>> @@ -1677,14 +1677,40 @@ static void igc_rx_checksum(struct igc_ring *ring,
>> le32_to_cpu(rx_desc->wb.upper.status_error));
>> }
>>
>> +/* Mapping HW RSS Type to enum pkt_hash_types */
>> +struct igc_rss_type {
>> + u8 hash_type; /* can contain enum pkt_hash_types */
>
> Why make a struct for one field? + short type note
>
>> +} igc_rss_type_table[IGC_RSS_TYPE_MAX_TABLE] = {
>> + [IGC_RSS_TYPE_NO_HASH].hash_type = PKT_HASH_TYPE_L2,
>> + [IGC_RSS_TYPE_HASH_TCP_IPV4].hash_type = PKT_HASH_TYPE_L4,
>> + [IGC_RSS_TYPE_HASH_IPV4].hash_type = PKT_HASH_TYPE_L3,
>> + [IGC_RSS_TYPE_HASH_TCP_IPV6].hash_type = PKT_HASH_TYPE_L4,
>> + [IGC_RSS_TYPE_HASH_IPV6_EX].hash_type = PKT_HASH_TYPE_L3,
>> + [IGC_RSS_TYPE_HASH_IPV6].hash_type = PKT_HASH_TYPE_L3,
>> + [IGC_RSS_TYPE_HASH_TCP_IPV6_EX].hash_type = PKT_HASH_TYPE_L4,
>> + [IGC_RSS_TYPE_HASH_UDP_IPV4].hash_type = PKT_HASH_TYPE_L4,
>> + [IGC_RSS_TYPE_HASH_UDP_IPV6].hash_type = PKT_HASH_TYPE_L4,
>> + [IGC_RSS_TYPE_HASH_UDP_IPV6_EX].hash_type = PKT_HASH_TYPE_L4,
>> + [10].hash_type = PKT_HASH_TYPE_L2, /* RSS Type above 9 "Reserved" by HW */
>> + [11].hash_type = PKT_HASH_TYPE_L2,
>> + [12].hash_type = PKT_HASH_TYPE_L2,
>> + [13].hash_type = PKT_HASH_TYPE_L2,
>> + [14].hash_type = PKT_HASH_TYPE_L2,
>> + [15].hash_type = PKT_HASH_TYPE_L2,
>
> Why define those empty if you could do a bound check in the code
> instead? E.g. `if (unlikely(bigger_than_9)) return PKT_HASH_TYPE_L2`.
Having a branch for this is likely slower. On godbolt I see that this
generates suboptimal and larger code.
>> +};
>> +
>> 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) {
>
> if (!(feature & HASH))
> return;
>
> and -1 indent level?
Usually, yes, I also prefer early return style code.
For one I just followed the existing style.
Second, I tried to code it up, but it looks ugly in this case, as the
variable defines need to get moved outside the if statement.
>> + u32 rss_hash = le32_to_cpu(rx_desc->wb.lower.hi_dword.rss);
>> + u8 rss_type = igc_rss_type(rx_desc);
>> + enum pkt_hash_types hash_type;
>> +
>> + hash_type = igc_rss_type_table[rss_type].hash_type;
>> + skb_set_hash(skb, rss_hash, hash_type);
>> + }
>> }
>
> [...]
>
> Thanks,
> Olek
>
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