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

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On sam., 2016-02-27 at 15:33 -0800, Peter Hurley wrote:
> On 02/27/2016 03:04 PM, David Miller wrote:
> > From: Peter Hurley <peter@hurleysoftware.com>
> > Date: Sat, 27 Feb 2016 12:29:39 -0800
> > 
> >> Not really. softirq raised from interrupt context will always execute
> >> on this cpu and not in ksoftirqd, unless load forces softirq loop abort.
> > 
> > That guarantee never was specified.
> 
> ??
> 
> Neither is running network socket servers at normal priority as if they're
> higher priority than softirq.
> 
> 
> > Or are you saying that by design, on a system under load, your UART
> > will not function properly?
> > 
> > Surely you don't mean that.
> 
> No, that's not what I mean.
> 
> What I mean is that bypassing the entire SOFTIRQ priority so that
> sshd can process one network packet makes a mockery of the point of softirq.
> 
> This hack to workaround NET_RX looping over-and-over-and-over affects every
> subsystem, not just one uart.
> 
> HI, TIMER, BLOCK; all of these are skipped: that's straight-up, a bug.

No idea what you talk about.

All pending softirq interrupts are processed. _Nothing_ is skipped.

Really, your system stability seems to depend on a completely
undocumented behavior of linux kernels before linux-3.8

If I understood, you expect that a tasklet activated from a softirq
handler is run from the same  __do_softirq() loop. This never has been
the case.

My change simply triggers the bug in your driver earlier. As David
pointed out, your bug should trigger the same on a loaded machine, even
if you revert my patch.

I honestly do not know why you arm a tasklet from NET_RX, why don't you
simply process this directly, so that you do not rely on some scheduler
decision ?

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Thread

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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