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

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Date:   Fri, 12 Jan 2018 18:44:09 +0100
From:   Mike Galbraith <efault@....de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 09:26 -0800, Linus Torvalds wrote:
> On Fri, Jan 12, 2018 at 2:13 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Fri, Jan 12, 2018 at 06:23:08AM +0100, Mike Galbraith wrote:
> >>
> >> How would that be better than what RT used to do, and I still do for my
> >> RT kernels via boot option, namely split ksoftirqd into per-softirq
> >> threads.
> >
> > Since we mention it; one of the problems RT has is that all RX is
> > through a single softirq context, which generates a priority inversion
> > between devices.
> 
> Oh, yes, that is complete and utter shit. Not acceptable at all.
> 
> As mentioned, it really would need to be per-cpu _and_ per-softirq.
> 
> Which is why I thought workqueues might be the thing. Whatever RT is
> doing is  apparently just pure and utter garbage.

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.

	-Mike