Subject: Re: Softirq priority inversion from "softirq: reduce latencies"

From: Peter Hurley <[email-masked]>
Date: Mon, 29 Feb 2016 11:13:36 -0800

On 02/29/2016 07:27 AM, Eric Dumazet wrote:
> On lun., 2016-02-29 at 07:03 -0800, Peter Hurley wrote:
> 
>> The reason why Eric's change is so effective for Eric's workload is
>> that it fixes the problem where NET_RX keeps getting new network packets
>> so it keeps looping, servicing more NET_RX softirq.
> 
> You have very little idea of what is happening in networking land.

While that is true, I can read a trace:

  ** already in NET_RX softirq **

  <idle>-0       0..s2   15us : kmem_cache_alloc: call_site=c08378e4 ptr=de55d7c0 bytes_req=192 bytes_alloc=192 gfp_flags=GFP_ATOMIC
  <idle>-0       0..s2   23us : netif_receive_skb_entry: dev=eth0 napi_id=0x0 queue_mapping=0 skbaddr=dca04400 vlan_tagged=0 vlan_proto=0x0000 vlan_tci=0x000
0 protocol=0x0800 ip_summed=0 hash=0x00000000 l4_hash=0 len=88 data_len=0 truesize=1984 mac_header_valid=1 mac_header=-14 nr_frags=0 gso_size=0 gso_type=0x0
  <idle>-0       0..s2   30us+: netif_receive_skb: dev=eth0 skbaddr=dca04400 len=88
  <idle>-0       0d.s5   98us : sched_waking: comm=sshd pid=750 prio=120 target_cpu=000
  <idle>-0       0d.s6  105us : sched_stat_sleep: comm=sshd pid=750 delay=3125230447 [ns]
  <idle>-0       0dns6  110us+: sched_wakeup: comm=sshd pid=750 prio=120 target_cpu=000
  <idle>-0       0dns4  123us+: timer_start: timer=dc940e9c function=tcp_delack_timer expires=9746 [timeout=10] flags=0x00000000
  <idle>-0       0dnH3  150us : irq_handler_entry: irq=176 name=4a100000.ethernet
  <idle>-0       0dnH3  153us : softirq_raise: vec=3 [action=NET_RX]
  <idle>-0       0dnH3  155us : irq_handler_exit: irq=176 ret=handled
  <idle>-0       0dnH3  160us : irq_handler_entry: irq=20 name=49000000.edma_ccint
  <idle>-0       0dnH3  163us : irq_handler_exit: irq=20 ret=handled
  <idle>-0       0.ns2  169us : napi_poll: napi poll on napi struct de465c30 for device eth0
  <idle>-0       0.ns2  171us : softirq_exit: vec=3 [action=NET_RX]

As you can see, NET_RX softirq is re-raised while in NET_RX softirq,
as a result of receiving new packets. So NET_RX will keep looping,
which is what I wrote.

> Once hard irq for RX has triggered, we arm a NAPI (NET_RX softirq), and
> no more irq will come unless the napi handler ran. Then when NAPI is
> complete, we re-allow interrupt to be delivered when a new packet is
> coming.
> 
> Yes, ksoftirqd runs under load, and this is _wanted_.
> 
> Sure, it might add a latency if some high prio task is wanting the same
> cpu, but this is exactly the purpose of having multi tasking.
> 
>