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Groups > linux.kernel > #1346103
| From | Peter Hurley <peter@hurleysoftware.com> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: Softirq priority inversion from "softirq: reduce latencies" |
| Date | 2016-02-29 20:20 +0100 |
| Message-ID | <r7Bwm-79s-5@gated-at.bofh.it> (permalink) |
| References | (1 earlier) <r72yB-6lb-1@gated-at.bofh.it> <r7d0Z-5MI-7@gated-at.bofh.it> <r7o66-5A0-5@gated-at.bofh.it> <r7xCs-4FN-45@gated-at.bofh.it> <r7xVM-4Nd-23@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
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. > >
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Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-27 19:20 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-27 21:20 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-27 21:40 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" David Miller <davem@davemloft.net> - 2016-02-28 00:10 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-28 00:40 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-28 03:10 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-28 03:20 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-28 03:20 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" David Miller <davem@davemloft.net> - 2016-02-28 05:50 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-02-28 07:00 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Francois Romieu <romieu@fr.zoreil.com> - 2016-02-28 18:10 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-02-29 06:00 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-29 16:10 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-29 16:30 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-29 17:00 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-29 17:30 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-29 19:10 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-29 19:30 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-29 20:00 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Thomas Gleixner <tglx@linutronix.de> - 2016-02-29 20:20 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" David Miller <davem@davemloft.net> - 2016-02-29 21:30 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-03-01 00:10 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-29 16:30 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-29 20:20 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-29 20:50 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Mike Galbraith <umgwanakikbuti@gmail.com> - 2016-02-29 16:50 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Peter Hurley <peter@hurleysoftware.com> - 2016-02-29 17:00 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-29 17:30 +0100
Re: Softirq priority inversion from "softirq: reduce latencies" David Miller <davem@davemloft.net> - 2016-02-29 18:20 +0100
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