Message-ID: <1515780912.21898.45.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>
Cc: Peter Zijlstra <peterz@infradead.org>,
        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 19:15:12 +0100
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On Fri, 2018-01-12 at 09:51 -0800, Linus Torvalds wrote:
> On Fri, Jan 12, 2018 at 9:44 AM, Mike Galbraith <efault@gmx.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.

Yeah, it is excessive more often than not.  You get to prioritize, and
segregate, which is nice, but you pay for it.

	-Mike