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

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Date:   Fri, 12 Jan 2018 20:28:06 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Mike Galbraith <efault@....de>,
        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>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context

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.

Thanks,

	tglx