Messages in this thread | | | Date | Wed, 13 Mar 2024 11:42:24 -0700 | Subject | Re: [PATCH net-next v4] net: Re-use and set mono_delivery_time bit for userspace tstamp packets | From | Martin KaFai Lau <> |
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On 3/13/24 1:52 AM, Willem de Bruijn wrote: > Martin KaFai Lau wrote: >> On 3/1/24 12:13 PM, Abhishek Chauhan wrote: >>> Bridge driver today has no support to forward the userspace timestamp >>> packets and ends up resetting the timestamp. ETF qdisc checks the >>> packet coming from userspace and encounters to be 0 thereby dropping >>> time sensitive packets. These changes will allow userspace timestamps >>> packets to be forwarded from the bridge to NIC drivers. >>> >>> Setting the same bit (mono_delivery_time) to avoid dropping of >>> userspace tstamp packets in the forwarding path. >>> >>> Existing functionality of mono_delivery_time remains unaltered here, >>> instead just extended with userspace tstamp support for bridge >>> forwarding path. >> >> The patch currently broke the bpf selftest test_tc_dtime: >> https://github.com/kernel-patches/bpf/actions/runs/8242487344/job/22541746675 >> >> In particular, there is a uapi field __sk_buff->tstamp_type which currently has >> BPF_SKB_TSTAMP_DELIVERY_MONO to mean skb->tstamp has the MONO "delivery" time. >> BPF_SKB_TSTAMP_UNSPEC means everything else (this could be a rx timestamp at >> ingress or a delivery time set by user space). >> >> __sk_buff->tstamp_type depends on skb->mono_delivery_time which does not >> necessarily mean mono after this patch. I thought about fixing it on the bpf >> side such that reading __sk_buff->tstamp_type only returns >> BPF_SKB_TSTAMP_DELIVERY_MONO when the skb->mono_delivery_time is set and skb->sk >> is IPPROTO_TCP. However, it won't work because of bpf_skb_set_tstamp(). >> >> There is a bpf helper, bpf_skb_set_tstamp(skb, tstamp, >> BPF_SKB_TSTAMP_DELIVERY_MONO). This helper changes both the skb->tstamp and the >> skb->mono_delivery_time. The expectation is this could change skb->tstamp in the >> ingress skb and redirect to egress sch_fq. It could also set a mono time to >> skb->tstamp where the udp sk->sk_clockid may not be necessary in mono and then >> bpf_redirect to egress sch_fq. When bpf_skb_set_tstamp(skb, tstamp, >> BPF_SKB_TSTAMP_DELIVERY_MONO) succeeds, reading __sk_buff->tstamp_type expects >> BPF_SKB_TSTAMP_DELIVERY_MONO also. >> >> I ran out of idea to solve this uapi breakage. >> >> I am afraid it may need to go back to v1 idea and use another bit >> (user_delivery_time) in the skb. > > Is the only conflict when bpf_skb_set_tstamp is called for an skb from > a socket with sk_clockid set (and thus SO_TXTIME called)?
Right, because skb->mono_delivery_time does not mean skb->tstamp is mono now and its interpretation depends on skb->sk->sk_clockid.
> Interpreting skb->tstamp as mono if skb->mono_delivery_time is set and > skb->sk is NULL is fine. This is the ingress to egress redirect case.
skb->sk == NULL is fine. I tried something like this in bpf_convert_tstamp_type_read() for reading __sk_buff->tstamp_type:
__sk_buff->tstamp_type is BPF_SKB_TSTAMP_DELIVERY_MONO when:
skb->mono_delivery_time == 1 && (!skb->sk || !sk_fullsock(skb->sk) /* tcp tw or req sk */ || skb->sk->sk_protocol == IPPROTO_TCP)
Not a small bpf instruction addition to bpf_convert_tstamp_type_read() but doable.
> > I don't see an immediate use for this BPF function on egress where it > would overwrite an SO_TXTIME timestamp and now skb->tstamp is mono, > but skb->sk != NULL and skb->sk->sk_clockid != CLOCK_MONOTONIC.
The bpf prog may act as a traffic shaper that limits the bandwidth usage of all outgoing packets (tcp/udp/...) by setting the mono EDT in skb->tstamp before sending to the sch_fq.
I currently also don't have a use case for skb->sk->sk_clockid != CLOCK_MONOTONIC. However, it is something that bpf_skb_set_tstamp() can do now before queuing to sch_fq.
The container (in netns + veth) may use other sk_clockid/qdisc (e.g. sch_etf) setup and the non mono skb->tstamp is not cleared now during dev_forward_skb() between the veth pair.
> > Perhaps bpf_skb_set_tstamp() can just fail if another delivery time is > already explicitly programmed?
This will change the existing bpf_skb_set_tstamp() behavior, so probably not acceptable.
> > skb->sk && > sock_flag(sk, SOCK_TXTIME) && > skb->sk->sk_clockid != CLOCK_MONOTONIC
> Either that, or unset SOCK_TXTIME to make sk_clockid undefined and > fall back on interpreting as monotonic.
Change sk->sk_flags in tc bpf prog? hmm... I am not sure this will work well also.
sock_valbool_flag(SOCK_TXTIME) should require a lock_sock() to make changes. The tc bpf prog cannot take the lock_sock, so bpf_skb_set_tstamp() currently only changes skb and does not change skb->sk.
I think changing sock_valbool_flag(SOCK_TXTIME) will also have a new user space visible side effect. The sendmsg for cmsg with SCM_TXTIME will start failing from looking at __sock_cmsg_send().
There may be a short period of disconnect between what is in sk->sk_flags and what is set in skb->tstamp. e.g. what if user space does setsockopt(SO_TXTIME) again after skb->tstamp is set by bpf. This could be considered a small glitch for some amount of skb(s) until the user space settled on setsockopt(SO_TXTIME).
I think all this is crying for another bit in skb to mean user_delivery_time (skb->tstamp depends on skb->sk->sk_clockid) while mono_delivery_time is the mono time either set by kernel-tcp or bpf. If we need to revert the mono_delivery_time reuse patch later, we will need to revert the netdev patch and the (to-be-made) bpf patch.
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