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

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Date:   Thu, 11 Jan 2018 12:22:38 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        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 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?

Would something like that sound sane?

Just a SMOP/SMOT (small matter of programming/testing).

               Linus