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

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Date:   Fri, 12 Jan 2018 16:15:04 +0100
From:   Mike Galbraith <efault@....de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        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, 2018-01-12 at 15:58 +0100, Frederic Weisbecker wrote:
> 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.
> 
> Workqueue are probably more simple. Unless you need to set specific prios
> to your ksoftirqds? Not sure if that's tunable on workqueues.

No, you can't prioritize workqueues, and they spawn threads whenever
they bloody well feel like.

I carry a hack to give users minimal control over kthread/workqueue
priority.  Very handy thing to have, especially if you're doing high
utilization stuff, and would prefer your box actually survive it.

	-Mike