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

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Date:   Thu, 11 Jan 2018 12:37:37 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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 12:34 PM, Dmitry Safonov <dima@...sta.com> 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?
>>
>> Would something like that sound sane?
>>
>> Just a SMOP/SMOT (small matter of programming/testing).
>
> I could try to write a PoC for that..
> What should be the trigger to fall into workqueue?
> How to tell if there're too many softirqs of the kind?
> Current logic with if (pending) in the end of __do_softirq()
> looks working selectively..
> It looks to be still possible to starve a cpu.

I guess we would need to track amount of time spent while processing
sortirq (while interrupting a non idle task)