Re: Lightweight packet timestamping

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Wed, 10 Jun 2020 23:09:34 +0200
Toke Høiland-Jørgensen <toke@redhat.com> wrote:

> Federico Parola <fede.parola@hotmail.it> writes:
> 
> > On 06/06/20 01:34, David Ahern wrote:  
> >> On 6/4/20 7:30 AM, Federico Parola wrote:  
> >>> Hello everybody,
>
> >>> I'm implementing a token bucket algorithm to apply rate limit to
> >>> traffic and I need the timestamp of packets to update the bucket.
> >>> To get this information I'm using the bpf_ktime_get_ns() helper
> >>> but I've discovered it has a non negligible impact on
> >>> performance. I've seen there is work in progress to make hardware
> >>> timestamps available to XDP programs, but I don't know if this
> >>> feature is already available. Is there a faster way to retrieve
> >>> this information?
>
> >>> Thanks for your attention.
> >>>  
> >> bpf_ktime_get_ns should be fairly light. What kind of performance loss
> >> are you seeing with it?  
> >
> > I've run some tests on a program forwarding packets between two 
> > interfaces and applying rate limit: using the bpf_ktime_get_ns() I can 
> > process up to 3.84 Mpps, if I replace the helper with a lookup on a map 
> > containing the current timestamp updated in user space I go up to 4.48
> > Mpps.

((1/3.84*1000)-(1/4.48*1000) = 37.20 ns overhead)

I was about to suggest doing something close to this.  That is, only call
bpf_ktime_get_ns() once per NAPI poll-cycle, and store the timestamp in
a map.  If you don't need super high per packet precision.  You can
even use a per-CPU map to store the info (to avoid cross CPU
cache/talk), because softirq will keep RX-processing pinned to a CPU.

It sounds like you update the timestamp from userspace, is that true?
(Quote: "current timestamp updated in user space")

I would suggest that you can leverage the softirq tracepoints (use
SEC("raw_tracepoint/") for low overhead).  E.g. irq:softirq_entry
(see when kernel calls trace_softirq_entry) to update the map once per
NAPI/net_rx_action. I have a bpftrace based-tool[1] that measure
network-softirq latency, e.g time it takes from "softirq_raise" until
it is run "softirq_entry".  You can leverage ideas from that script,
like 'vec == 3' is NET_RX_SOFTIRQ to limit this to networking.

[1] https://github.com/xdp-project/xdp-project/blob/master/areas/latency/softirq_net_latency.bt

> Can you share more details on the platform you're running this on?
> I.e., CPU and chipset details, network driver, etc.

Yes, please.  I plan to work on XDP-feature of extracting hardware
offload-info from the drivers descriptor, like timestamps, vlan,
rss-hash, checksum, etc.  If you tell me what NIC driver you are using,
I could make sure to include that in the supported drivers.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer