John Fastabend wrote: > Andrii Nakryiko wrote: > > On Mon, Sep 11, 2023 at 7:43 AM John Fastabend <john.fastabend@xxxxxxxxx> wrote: > > > > > > Andrii Nakryiko wrote: > > > > On Sun, Jul 16, 2023 at 9:37 PM John Fastabend <john.fastabend@xxxxxxxxx> wrote: > > > > > > > > > > Andrii Nakryiko wrote: > > > > > > Hey John, > > > > > > > > > > Sorry missed this while I was on PTO that week. > > > > > > > > yeah, vacations tend to cause missing things :) > > > > > > > > > > > > > > > > > > > > > We've been recently experimenting with using BPF_SK_SKB_STREAM_PARSER > > > > > > and BPF_SK_SKB_STREAM_VERDICT with sockmap/sockhash to perform > > > > > > in-kernel parsing of RSocket frames. A very simple format ([0]) where > > > > > > the first 3 bytes specify the size of the frame payload. The idea was > > > > > > to collect the entire frame in the kernel before notifying user-space > > > > > > that data is available. This is meant to minimize unnecessary wakeups > > > > > > due to incomplete logical frames, saving CPU. > > > > > > > > > > Nice. > > > > > > > > > > > > > > > > > You can find the BPF source code I've used at [1], it has lots of > > > > > > extra logging and stuff, but the idea is to read the first 3 bytes of > > > > > > each logical frame, and return the expected full frame size from the > > > > > > parser program. The verdict program always just returns SK_PASS. > > > > > > > > > > > > This seems to work exactly as expected in manual simulations of > > > > > > various packet size distributions, and even for a bunch of > > > > > > ping/pong-like benchmark (which are very sensitive to correct frame > > > > > > length determination, so I'm reasonably confident we don't screw that > > > > > > up much). And yet, when benchmarking sending multiple logical RPC > > > > > > streams over the same single socket (so many interleaving RSocket > > > > > > frames on single socket, but in terms of logical frames nothing should > > > > > > change), we often see that while full frame hasn't been accumulated in > > > > > > socket receive buffer yet, epoll_wait() for that socket would return > > > > > > with success notifying user space that there is data on socket. > > > > > > Subsequent recvfrom() call would immediately return -EAGAIN and no > > > > > > data, and our benchmark would go on this loop of useless > > > > > > epoll_wait()+recvfrom() calls back to back, many times over. > > > > > > > > > > Aha yes this sounds bad. > > > > > > > > > > > > > > > > > So I have a few questions: > > > > > > - is the above use case something that was meant to be handled by > > > > > > sockmap+parser/verdict? > > > > > > > > > > We shouldn't wake up user space if there is nothing to read. So > > > > > yes this seems like a valid use case to me. > > > > > > > > > > > - is it correct to assume that epoll won't wake up until amount of > > > > > > bytes requested by parser program is accumulated (this seems to be the > > > > > > case from manually experimenting with various "packet delays"); > > > > > > > > > > Seems there is some bug that races and causes it to wake up > > > > > user space. I'm aware of a couple bugs in the stream parser > > > > > that I wanted to fix. Not sure I can get to them this week > > > > > but should have time next week. We have a couple more fixes > > > > > to resolve a few HTTPS server compliance tests as well. > > > > > > > > > > > - is there some known bug or race in how sockmap and strparser > > > > > > framework interacts with epoll subsystem that could cause this weird > > > > > > epoll_wait() behavior? > > > > > > > > > > Yes I know of some races in strparser. I'll elaborate later > > > > > probably with patches as I don't recall them readily at the > > > > > moment. > > > > > > > > So I missed a good chunk of BPF mailing list traffic while I was on my > > > > PTO. Did you end up getting to these bugs in strparser logic? Should I > > > > try running the latest bpf-next/net-next on our production workload to > > > > see if this is still happening? > > > > > > You will likely still hit there error I haven't got it out of my queue > > > yet. I just knocked off a couple things last week so could probably > > > take a look at flushing my queue this week. Then it would make sense > > > to retest to see if its something new or not. > > > > > > I'll at least send an RFC with the idea even if I don't get to testing > > > it yet. > > > > Sounds good, thanks a lot! > > > > > > > > Thanks, > > > John > > Hi Andrii, > > Finally got around to thinking about this. And also I belive we have > the verdict programs mostly fixed up to handle polling correctly now. > The problem was incorrectly handling the tcp_sock copied_seq var > which is used by tcp_epollin_ready() to wakeup the application. Its > also used to calculate responses to some ioctl we found servers using > to decide when to actually do a recv, e.g. they wait on the ioctl until > enough bytes are received. > > The trick is to ensure we only update copied_seq when the bytes are > in fact actually ready to read from socket queue. The sockmap verdict > program code was incrementing this before running the verdict prog > so we raced with userspace. It kind of works in many cases because > we are holding the sock lock in many cases so we block the user space > recvmsg. > > Now to your problem as I understand it. You are trying to use the > parser program to hold some N bytes where N is the message block. > At which point it will get pushed to a verdict prog and finally > queued in the msg recieve queue so a syscall to recv*() can > actually read it. The parser program, unlike if you just have > a verdict prog, causes the skb to run through the stream parser to > collect bytes and then run the verdict program. The stream parser > is using tcp_read_sock() which increments the seq_copied immediately > even before the verdict prog is run so I expect the odd behavior > you see is when that race completes. It likely mostly works because > we have the sock lock for lots of the code making the race behavior > smaller than it might otherwise appear. I didn't do a full anlaysis > but it might just be when we hit an ENOMEM condition and need to > backoff. Which might explain why you only see the issue when you > run with larger envs. > > It feels a bit suboptimal in your case to run two BPF programs and > parser logic compared to a single verdict program. Could we just > add a bpf helper we can run from the verdict program to only wake > up the user space after N bytes. To mirror the sk_msg programs we > migth call it bpf_skb_cork_bytes(skb, bytes, flags). We could use > flags to decide if we need to call the prog again with the new > full sized skb or if we can just process it directly without the > extra BPF call. > > This with the other piece we want from our side to allow running > verdict and sk_msg programs on sockets without having them in a > sockmap/sockhash it would seem like a better system to me. The > idea to drop the sockmap/sockhash is because we never remove progs > once they are added and we add them from sockops side. The filter > to socketes is almost always the port + metadata related to the > process or environment. This simplifies having to manage the > sockmap/sockhash and guess what size it should be. Sometimes we > overrun these maps and have to kill connections until we can > get more space. > > For you case I would expect it to be (a) simpler just a single > program to manage instead of two and a map and (b) more efficient > to call one prog in datapath vs two. > > WDYT? > > Thanks, > John On second thought I'll also fix the existing stream parser code here shortly. Its a bit broken if I just leave it as is, but I still like the idea of a new helper.
