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

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Message-ID: <20180112101336.GT3040@hirez.programming.kicks-ass.net>
Date:   Fri, 12 Jan 2018 11:13:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Galbraith <efault@....de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.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, Jan 12, 2018 at 06:23:08AM +0100, Mike Galbraith wrote:
> On Thu, 2018-01-11 at 12:22 -0800, Linus Torvalds wrote:
> > On Thu, Jan 11, 2018 at 12:16 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > Note that when I implemented TCP Small queues, I did experiments between
> > > using a work queue or a tasklet, and workqueues added unacceptable P99
> > > latencies, when many user threads are competing with kernel threads.
> > 
> > Yes.
> > 
> > So I think one solution might be to have a hybrid system, where we do
> > the softirq's synchronously normally (which is what you really want
> > for good latency).
> > 
> > But then fall down on a threaded model - but that fallback case should
> > be per-softirq, not global. So if one softirq uses a lot of CPU time,
> > that shouldn't affect the latency of other softirqs.
> > 
> > So maybe we could get rid of the per-cpu ksoftirqd entirely, and
> > replace it with with per-cpu and per-softirq workqueues?
> 
> 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.

If we're splitting things, it would be awesome if we could get
per-device context.