Subject: Re: [PATCH net-next 15/24] net: Use nested-BH locking for XDP redirect.

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 17 Jan 2024 17:37:29 +0100

Sebastian Andrzej Siewior <[email-masked]> writes:

> On 2024-01-04 20:29:02 [+0100], Toke Høiland-Jørgensen wrote:
>> Alexei Starovoitov <[email-masked]> writes:
>> 
>> >> @@ -3925,6 +3926,7 @@ struct sk_buff *tcf_qevent_handle(struct tcf_qevent *qe, struct Qdisc *sch, stru
>> >>
>> >>         fl = rcu_dereference_bh(qe->filter_chain);
>> >>
>> >> +       guard(local_lock_nested_bh)(&bpf_run_lock.redirect_lock);
>> >>         switch (tcf_classify(skb, NULL, fl, &cl_res, false)) {
>> >>         case TC_ACT_SHOT:
>> >>                 qdisc_qstats_drop(sch);
>> >
>> > Here and in all other places this patch adds locks that
>> > will kill performance of XDP, tcx and everything else in networking.
>> >
>> > I'm surprised Jesper and other folks are not jumping in with nacks.
>> > We measure performance in nanoseconds here.
>> > Extra lock is no go.
>> > Please find a different way without ruining performance.
>> 
>> I'll add that while all this compiles out as no-ops on !PREEMPT_RT, I do
>> believe there are people who are using XDP on PREEMPT_RT kernels and
>> still expect decent performance. And to achieve that it is absolutely
>> imperative that we can amortise expensive operations (such as locking)
>> over multiple packets.
>> 
>> I realise there's a fundamental trade-off between the amount of
>> amortisation and the latency hit that we take from holding locks for
>> longer, but tuning the batch size (while still keeping some amount of
>> batching) may be a way forward? I suppose Jakub's suggestion in the
>> other part of the thread, of putting the locks around napi->poll(), is a
>> step towards something like this.
>
> The RT requirements are usually different. Networking as in CAN might be
> important but Ethernet could only used for remote communication and so
> "not" important. People complained that they need to wait for Ethernet
> to be done until the CAN packet can be injected into the stack.
> With that expectation you would like to pause Ethernet immediately and
> switch over the CAN interrupt thread.
>
> But if someone managed to setup XDP then it is likely to be important.
> With RT traffic it is usually not the throughput that matters but the
> latency. You are likely in the position to receive a packet, say every
> 1ms, and need to respond immediately. XDP would be used to inspect the
> packet and either hand it over to the stack or process it.

I am not contesting that latency is important, but it's a pretty
fundamental trade-off and we don't want to kill throughput entirely
either. Especially since this is global to the whole kernel; and there
are definitely people who want to use XDP on an RT kernel and still
achieve high PPS rates.

(Whether those people really strictly speaking need to be running an RT
kernel is maybe debatable, but it does happen).

> I expected the lock operation (under RT) to always succeeds and not
> cause any delay because it should not be contended.

A lock does cause delay even when it's not contended. Bear in mind that
at 10 Gbps line rate, we have a budget of 64 nanoseconds to process each
packet (for 64-byte packets). So just the atomic op to figure out
whether there's any contention (around 10ns on the Intel processors I
usually test on) will blow a huge chunk of the total processing budget.
We can't actually do the full processing needed in those 64 nanoseconds
(not to mention the 6.4 nanoseconds we have available at 100Gbps), which
is why it's essential to amortise as much as we can over multiple
packets.

This is all back-of-the-envelope calculations, of course. Having some
actual numbers to look at would be great; I don't suppose you have a
setup where you can run xdp-bench and see how your patches affect the
throughput?

> It should only block if something with higher priority preempted the
> current interrupt thread _and_ also happen to use XDP on the same CPU.
> In that case (XDP is needed) it would flush the current user out of
> the locked section before the higher-prio thread could take over.
> Doing bulk and allowing the low-priority thread to complete would
> delay the high-priority thread. Maybe I am too pessimistic here and
> having two XDP programs on one CPU is unlikely to happen.
>
> Adding the lock on per-NAPI basis would allow to batch packets.
> Acquiring the lock only if XDP is supported would not block the CAN
> drivers since they dont't support XDP. But sounds like a hack.

I chatted with Jesper about this, and he had an idea not too far from
this: split up the XDP and regular stack processing in two stages, each
with their individual batching. So whereas right now we're doing
something like:

run_napi()
  bh_disable()
  for pkt in budget:
    act = run_xdp(pkt)
    if (act == XDP_PASS)
      run_netstack(pkt)  // this is the expensive bit
  bh_enable()

We could instead do:

run_napi()
  bh_disable()
  for pkt in budget:
    act = run_xdp(pkt)
    if (act == XDP_PASS)
      add_to_list(pkt, to_stack_list)
  bh_enable()
  // sched point
  bh_disable()
  for pkt in to_stack_list:
    run_netstack(pkt)
  bh_enable()

This would limit the batching that blocks everything to only the XDP
processing itself, which should limit the maximum time spent in the
blocking state significantly compared to what we have today. The caveat
being that rearranging things like this is potentially a pretty major
refactoring task that needs to touch all the drivers (even if some of
the logic can be moved into the core code in the process). So not really
sure if this approach is feasible, TBH.

> Daniel said netkit doesn't need this locking because it is not
> supporting this redirect and it made me think. Would it work to make
> the redirect structures part of the bpf_prog-structure instead of
> per-CPU? My understanding is that eBPF's programs data structures are
> part of it and contain locking allowing one eBPF program preempt
> another one.
> Having the redirect structures part of the program would obsolete
> locking. Do I miss anything?

This won't work, unfortunately: the same XDP program can be attached to
multiple interfaces simultaneously, and for hardware with multiple
receive queues (which is most of the hardware that supports XDP), it can
even run simultaneously on multiple CPUs on the same interface. This is
the reason why this is all being kept in per-CPU variables today.

-Toke