Re: [RFC PATCH 2/2] softirq: Drop the warning from do_softirq_post_smp_call_flush().

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 15/08/2023 14.08, Jesper Dangaard Brouer wrote:
> 
> 
> On 14/08/2023 11.35, Sebastian Andrzej Siewior wrote:
>> 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 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?


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

--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