linux-kernel - Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

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Date:   Fri, 12 Jan 2018 19:44:30 +0100
From:   Mike Galbraith <efault@....de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Dmitry Safonov <dima@...sta.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Radu Rendec <rrendec@...sta.com>,
        Rik van Riel <riel@...hat.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context

On Fri, 2018-01-12 at 19:15 +0100, Mike Galbraith wrote:
> On Fri, 2018-01-12 at 09:51 -0800, Linus Torvalds wrote:
> > On Fri, Jan 12, 2018 at 9:44 AM, Mike Galbraith <efault@....de> wrote:
> > >
> > > Nah, a misunderstanding happened.  RT that still offers full threading
> > > creates per-softirq threads per cpu.  The regular trees split ksoftirqd
> > > into only two threads per cpu, one processes timer/hrtimer softriqs,
> > > the other processes the rest.
> > 
> > Ok, that sounds like it should work, but it also sounds like it's very
> > specific to RT itself.
> > 
> > For example, the dvb issue was not about the timer softirqs, but about
> > the tasklet ones.
> > 
> > So maybe we wouldn't need to split it for _every_ softirq, but we'd
> > need to split it more than just along the timer case.
> > 
> > And it does sound a bit excessive to have ten fixed threads for every
> > CPU. The days when tens of CPU's meant "huge system" are gone. These
> > days it can be a phone.
> 
> Yeah, it is excessive more often than not.  You get to prioritize, and
> segregate, which is nice, but you pay for it.

BTW, much of the softirq load in RT is processed by the raising task.

      tbench_srv-6985  [000] d...112   293.902511: softirq_raise: vec=3 [action=NET_RX]
      tbench_srv-6985  [000] .....13   293.902511: softirq_entry: vec=3 [action=NET_RX]
      tbench_srv-6985  [000] .....13   293.902515: softirq_exit: vec=3 [action=NET_RX]
          tbench-6984  [003] d...112   293.902520: softirq_raise: vec=3 [action=NET_RX]
          tbench-6984  [003] .....13   293.902520: softirq_entry: vec=3 [action=NET_RX]
          tbench-6984  [003] .....13   293.902523: softirq_exit: vec=3 [action=NET_RX]