Message-ID: <1515779049.21898.23.camel@gmx.de>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context
From: Mike Galbraith <efault@gmx.de>
To: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc: Eric Dumazet <edumazet@google.com>,
        Dmitry Safonov <dima@arista.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Miller <davem@davemloft.net>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@verizon.com>,
        Paolo Abeni <pabeni@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Radu Rendec <rrendec@arista.com>,
        Rik van Riel <riel@redhat.com>,
        Stanislaw Gruszka <sgruszka@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Wanpeng Li <wanpeng.li@hotmail.com>
Date: Fri, 12 Jan 2018 18:44:09 +0100
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On Fri, 2018-01-12 at 09:26 -0800, Linus Torvalds wrote:
> On Fri, Jan 12, 2018 at 2:13 AM, Peter Zijlstra <peterz@infradead.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