Messages in this thread | | | Date | Tue, 10 Oct 2023 14:51:17 +0900 | Subject | Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature | From | Akihiko Odaki <> |
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On 2023/10/10 14:45, Jason Wang wrote: > On Tue, Oct 10, 2023 at 9:52 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >> >> On 2023/10/09 19:44, Willem de Bruijn wrote: >>> On Mon, Oct 9, 2023 at 3:12 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >>>> >>>> On 2023/10/09 19:06, Willem de Bruijn wrote: >>>>> On Mon, Oct 9, 2023 at 3:02 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >>>>>> >>>>>> On 2023/10/09 18:57, Willem de Bruijn wrote: >>>>>>> On Mon, Oct 9, 2023 at 3:57 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >>>>>>>> >>>>>>>> On 2023/10/09 17:04, Willem de Bruijn wrote: >>>>>>>>> On Sun, Oct 8, 2023 at 3:46 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >>>>>>>>>> >>>>>>>>>> On 2023/10/09 5:08, Willem de Bruijn wrote: >>>>>>>>>>> On Sun, Oct 8, 2023 at 10:04 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >>>>>>>>>>>> >>>>>>>>>>>> On 2023/10/09 4:07, Willem de Bruijn wrote: >>>>>>>>>>>>> On Sun, Oct 8, 2023 at 7:22 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>> virtio-net have two usage of hashes: one is RSS and another is hash >>>>>>>>>>>>>> reporting. Conventionally the hash calculation was done by the VMM. >>>>>>>>>>>>>> However, computing the hash after the queue was chosen defeats the >>>>>>>>>>>>>> purpose of RSS. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Another approach is to use eBPF steering program. This approach has >>>>>>>>>>>>>> another downside: it cannot report the calculated hash due to the >>>>>>>>>>>>>> restrictive nature of eBPF. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Introduce the code to compute hashes to the kernel in order to overcome >>>>>>>>>>>>>> thse challenges. An alternative solution is to extend the eBPF steering >>>>>>>>>>>>>> program so that it will be able to report to the userspace, but it makes >>>>>>>>>>>>>> little sense to allow to implement different hashing algorithms with >>>>>>>>>>>>>> eBPF since the hash value reported by virtio-net is strictly defined by >>>>>>>>>>>>>> the specification. >>>>>>>>>>>>>> >>>>>>>>>>>>>> The hash value already stored in sk_buff is not used and computed >>>>>>>>>>>>>> independently since it may have been computed in a way not conformant >>>>>>>>>>>>>> with the specification. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> >>>>>>>>>>>>>> --- >>>>>>>>>>>>> >>>>>>>>>>>>>> +static const struct tun_vnet_hash_cap tun_vnet_hash_cap = { >>>>>>>>>>>>>> + .max_indirection_table_length = >>>>>>>>>>>>>> + TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH, >>>>>>>>>>>>>> + >>>>>>>>>>>>>> + .types = VIRTIO_NET_SUPPORTED_HASH_TYPES >>>>>>>>>>>>>> +}; >>>>>>>>>>>>> >>>>>>>>>>>>> No need to have explicit capabilities exchange like this? Tun either >>>>>>>>>>>>> supports all or none. >>>>>>>>>>>> >>>>>>>>>>>> tun does not support VIRTIO_NET_RSS_HASH_TYPE_IP_EX, >>>>>>>>>>>> VIRTIO_NET_RSS_HASH_TYPE_TCP_EX, and VIRTIO_NET_RSS_HASH_TYPE_UDP_EX. >>>>>>>>>>>> >>>>>>>>>>>> It is because the flow dissector does not support IPv6 extensions. The >>>>>>>>>>>> specification is also vague, and does not tell how many TLVs should be >>>>>>>>>>>> consumed at most when interpreting destination option header so I chose >>>>>>>>>>>> to avoid adding code for these hash types to the flow dissector. I doubt >>>>>>>>>>>> anyone will complain about it since nobody complains for Linux. >>>>>>>>>>>> >>>>>>>>>>>> I'm also adding this so that we can extend it later. >>>>>>>>>>>> max_indirection_table_length may grow for systems with 128+ CPUs, or >>>>>>>>>>>> types may have other bits for new protocols in the future. >>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>>> case TUNSETSTEERINGEBPF: >>>>>>>>>>>>>> - ret = tun_set_ebpf(tun, &tun->steering_prog, argp); >>>>>>>>>>>>>> + bpf_ret = tun_set_ebpf(tun, &tun->steering_prog, argp); >>>>>>>>>>>>>> + if (IS_ERR(bpf_ret)) >>>>>>>>>>>>>> + ret = PTR_ERR(bpf_ret); >>>>>>>>>>>>>> + else if (bpf_ret) >>>>>>>>>>>>>> + tun->vnet_hash.flags &= ~TUN_VNET_HASH_RSS; >>>>>>>>>>>>> >>>>>>>>>>>>> Don't make one feature disable another. >>>>>>>>>>>>> >>>>>>>>>>>>> TUNSETSTEERINGEBPF and TUNSETVNETHASH are mutually exclusive >>>>>>>>>>>>> functions. If one is enabled the other call should fail, with EBUSY >>>>>>>>>>>>> for instance. >>>>>>>>>>>>> >>>>>>>>>>>>>> + case TUNSETVNETHASH: >>>>>>>>>>>>>> + len = sizeof(vnet_hash); >>>>>>>>>>>>>> + if (copy_from_user(&vnet_hash, argp, len)) { >>>>>>>>>>>>>> + ret = -EFAULT; >>>>>>>>>>>>>> + break; >>>>>>>>>>>>>> + } >>>>>>>>>>>>>> + >>>>>>>>>>>>>> + if (((vnet_hash.flags & TUN_VNET_HASH_REPORT) && >>>>>>>>>>>>>> + (tun->vnet_hdr_sz < sizeof(struct virtio_net_hdr_v1_hash) || >>>>>>>>>>>>>> + !tun_is_little_endian(tun))) || >>>>>>>>>>>>>> + vnet_hash.indirection_table_mask >= >>>>>>>>>>>>>> + TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH) { >>>>>>>>>>>>>> + ret = -EINVAL; >>>>>>>>>>>>>> + break; >>>>>>>>>>>>>> + } >>>>>>>>>>>>>> + >>>>>>>>>>>>>> + argp = (u8 __user *)argp + len; >>>>>>>>>>>>>> + len = (vnet_hash.indirection_table_mask + 1) * 2; >>>>>>>>>>>>>> + if (copy_from_user(vnet_hash_indirection_table, argp, len)) { >>>>>>>>>>>>>> + ret = -EFAULT; >>>>>>>>>>>>>> + break; >>>>>>>>>>>>>> + } >>>>>>>>>>>>>> + >>>>>>>>>>>>>> + argp = (u8 __user *)argp + len; >>>>>>>>>>>>>> + len = virtio_net_hash_key_length(vnet_hash.types); >>>>>>>>>>>>>> + >>>>>>>>>>>>>> + if (copy_from_user(vnet_hash_key, argp, len)) { >>>>>>>>>>>>>> + ret = -EFAULT; >>>>>>>>>>>>>> + break; >>>>>>>>>>>>>> + } >>>>>>>>>>>>> >>>>>>>>>>>>> Probably easier and less error-prone to define a fixed size control >>>>>>>>>>>>> struct with the max indirection table size. >>>>>>>>>>>> >>>>>>>>>>>> I made its size variable because the indirection table and key may grow >>>>>>>>>>>> in the future as I wrote above. >>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> Btw: please trim the CC: list considerably on future patches. >>>>>>>>>>>> >>>>>>>>>>>> I'll do so in the next version with the TUNSETSTEERINGEBPF change you >>>>>>>>>>>> proposed. >>>>>>>>>>> >>>>>>>>>>> To be clear: please don't just resubmit with that one change. >>>>>>>>>>> >>>>>>>>>>> The skb and cb issues are quite fundamental issues that need to be resolved. >>>>>>>>>>> >>>>>>>>>>> I'd like to understand why adjusting the existing BPF feature for this >>>>>>>>>>> exact purpose cannot be amended to return the key it produced. >>>>>>>>>> >>>>>>>>>> eBPF steering program is not designed for this particular problem in my >>>>>>>>>> understanding. It was introduced to derive hash values with an >>>>>>>>>> understanding of application-specific semantics of packets instead of >>>>>>>>>> generic IP/TCP/UDP semantics. >>>>>>>>>> >>>>>>>>>> This problem is rather different in terms that the hash derivation is >>>>>>>>>> strictly defined by virtio-net. I don't think it makes sense to >>>>>>>>>> introduce the complexity of BPF when you always run the same code. >>>>>>>>>> >>>>>>>>>> It can utilize the existing flow dissector and also make it easier to >>>>>>>>>> use for the userspace by implementing this in the kernel. >>>>>>>>> >>>>>>>>> Ok. There does appear to be overlap in functionality. But it might be >>>>>>>>> easier to deploy to just have standard Toeplitz available without >>>>>>>>> having to compile and load an eBPF program. >>>>>>>>> >>>>>>>>> As for the sk_buff and cb[] changes. The first is really not needed. >>>>>>>>> sk_buff simply would not scale if every edge case needs a few bits. >>>>>>>> >>>>>>>> An alternative is to move the bit to cb[] and clear it for every code >>>>>>>> paths that lead to ndo_start_xmit(), but I'm worried that it is error-prone. >>>>>>>> >>>>>>>> I think we can put the bit in sk_buff for now. We can implement the >>>>>>>> alternative when we are short of bits. >>>>>>> >>>>>>> I disagree. sk_buff fields add a cost to every code path. They cannot >>>>>>> be added for every edge case. >>>>>> >>>>>> It only takes an unused bit and does not grow the sk_buff size so I >>>>>> think it has practically no cost for now. >>>>> >>>>> The problem is that that thinking leads to death by a thousand cuts. >>>>> >>>>> "for now" forces the cost of having to think hard how to avoid growing >>>>> sk_buff onto the next person. Let's do it right from the start. >>>> >>>> I see. I described an alternative to move the bit to cb[] and clear it >>>> in all code paths that leads to ndo_start_xmit() earlier. Does that >>>> sound good to you? >>> >>> If you use the control block to pass information between >>> __dev_queue_xmit on the tun device and tun_net_xmit, using gso_skb_cb, >>> the field can be left undefined in all non-tun paths. tun_select_queue >>> can initialize. >> >> The problem is that tun_select_queue() is not always called. >> netdev_core_pick_tx() ensures dev->real_num_tx_queues != 1 before >> calling it, but this variable may change later and result in a race >> condition. Another case is that XDP with predefined queue. >> >>> >>> I would still use skb->hash to encode the hash. That hash type of that >>> field is not strictly defined. It can be siphash from ___skb_get_hash >>> or a device hash, which most likely also uses Toeplitz. Then you also >>> don't run into the problem of growing the struct size. >> >> I'm concerned exactly because it's not strictly defined. Someone may >> decide to overwrite it later if we are not cautious enough. qdisc_skb_cb >> also has sufficient space to contain both of the hash value and type. > > How about using skb extensions?
I think it will work. I'll try it in the next version.
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