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

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Date:   Fri, 12 Jan 2018 20:53:17 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Dmitry Safonov <dima@...sta.com>,
        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>,
        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 08:28:06PM +0100, Thomas Gleixner wrote:
> On Fri, 12 Jan 2018, 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.
> 
> That's true.
> 
> One thing which might worth a try is utilizing the threaded irq
> infrastructure and that might also pave the way to address Peters request
> for per device splitting. I've experimented with that in the past but never
> got around to finish it completely. I should have half baken patches
> somewhere in the poison cabinet.

I'll gladly have a look at them to see what I can do.

Thanks.