On Wed, Aug 16, 2023 at 9:49 AM Jesper Dangaard Brouer [off-list ref] wrote:

On 15/08/2023 14.08, Jesper Dangaard Brouer wrote:

quoted

On 14/08/2023 11.35, Sebastian Andrzej Siewior wrote:

quoted

This is an undesired situation and it has been attempted to avoid the
situation in which ksoftirqd becomes scheduled. This changed since
commit d15121be74856 ("Revert "softirq: Let ksoftirqd do its job"")
and now a threaded interrupt handler will handle soft interrupts at its
end even if ksoftirqd is pending. That means that they will be processed
in the context in which they were raised.

$ git describe --contains d15121be74856
v6.5-rc1~232^2~4

That revert basically removes the "overload" protection that was added
to cope with DDoS situations in Aug 2016 (Cc. Cloudflare).  As described
in https://git.kernel.org/torvalds/c/4cd13c21b207 ("softirq: Let
ksoftirqd do its job") in UDP overload situations when UDP socket
receiver runs on same CPU as ksoftirqd it "falls-off-an-edge" and almost
doesn't process packets (because softirq steals CPU/sched time from UDP
pid).  Warning Cloudflare (Cc) as this might affect their production
use-cases, and I recommend getting involved to evaluate the effect of
these changes.

I did some testing on net-next (with commit d15121be74856 ("Revert
"softirq: Let ksoftirqd do its job"") using UDP pktgen + udp_sink.

And I observe the old overload issue occur again, where userspace
process (udp_sink) process very few packets when running on *same* CPU
as the NAPI-RX/IRQ processing.  The perf report "comm" clearly shows
that NAPI runs in the context of the "udp_sink" process, stealing its
sched time. (Same CPU around 3Kpps and diff CPU 1722Kpps, see details
below).
What happens are that NAPI takes 64 packets and queue them to the
udp_sink process *socket*, the udp_sink process *wakeup* process 1
packet from socket queue and on exit (__local_bh_enable_ip) runs softirq
that starts NAPI (to again process 64 packets... repeat).

I think there are two scenarios to consider:
1. Actual DoS scenario. In this case, we would drop DoS packets
through XDP, which might actually relieve the stress. According to
Marek's blog XDP can indeed drop 10M pps [1] so it might not steal too
much time. This is also something I would like to validate again since
I cannot tell if those tests were performed before or after the
reverted commit.
2. Legit elephant flows (so it should not be just dropped). This one
is closer to what you tested above, and it is a much harder issue
since packets are legit and should not be dropped early at XDP. Let
the scheduler move affected processes away seems to be the non-optimal
but straight answer for now. However, I suspect this would impose an
overload issue for those programmed with RFS or ARFS, since flows
would "follow" the processes. They probably have to force threaded
NAPI for tuning.

[1] https://blog.cloudflare.com/how-to-drop-10-million-packets/

quoted

I do realize/acknowledge that the reverted patch caused other latency
issues, given it was a "big-hammer" approach affecting other softirq
processing (as can be seen by e.g. the watchdog fixes patches).
Thus, the revert makes sense, but how to regain the "overload"
protection such that RX networking cannot starve processes reading from
the socket? (is this what Sebastian's patchset does?)

I'm no expert in sched / softirq area of the kernel, but I'm willing to
help out testing different solution that can regain the "overload"
protection e.g. avoid packet processing "falls-of-an-edge" (and thus
opens the kernel to be DDoS'ed easily).
Is this what Sebastian's patchset does?

quoted

Thread link for people Cc'ed:
https://lore.kernel.org/all/20230814093528.117342-1-bigeasy@linutronix.de/#r (local)

--Jesper
(some testlab results below)

[udp_sink]
https://github.com/netoptimizer/network-testing/blob/master/src/udp_sink.c


When udp_sink runs on same CPU and NAPI/softirq
  - UdpInDatagrams: 2,948 packets/sec

$ nstat -n && sleep 1 && nstat
#kernel
IpInReceives                    2831056            0.0
IpInDelivers                    2831053            0.0
UdpInDatagrams                  2948               0.0
UdpInErrors                     2828118            0.0
UdpRcvbufErrors                 2828118            0.0
IpExtInOctets                   130206496          0.0
IpExtInNoECTPkts                2830576            0.0

When udp_sink runs on another CPU than NAPI-RX.
  - UdpInDatagrams: 1,722,307 pps

$ nstat -n && sleep 1 && nstat
#kernel
IpInReceives                    2318560            0.0
IpInDelivers                    2318562            0.0
UdpInDatagrams                  1722307            0.0
UdpInErrors                     596280             0.0
UdpRcvbufErrors                 596280             0.0
IpExtInOctets                   106634256          0.0
IpExtInNoECTPkts                2318136            0.0

-- 

Yan