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

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Date:   Wed, 10 Jan 2018 21:48:00 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, 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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Tue, 2018-01-09 at 10:02 -0800, Eric Dumazet wrote:
> On Tue, Jan 9, 2018 at 5:36 AM, Dmitry Safonov <dima@...sta.com>
> wrote:
> > Warning: Not merge-ready
> > 
> > I. Current workflow of ksoftirqd.
> >   Softirqs are processed in the context of ksoftirqd iff they are
> >   being raised very frequently. How it works:
> >   do_softirq() and invoke_softirq() deffer pending softirq iff
> > 
> 
> ...
> 
> > 
> >   Note, that I tested in VMs and I've found that if I produce more
> >   hw irqs on the host, than the results for master are not that
> >   dramatically bad, but still much worse then with RFC.
> >   By that reason I have qualms if my test's results are correct.
> 
> Note that deferring all NET RX/TX to ksoftirqd is going to
> dramatically hurt tail latencies.
> 
> You really should test with RPC like workloads (netperf -t TCP_RR)
> and
> hundred of threads per cpu :/

Yeah, thanks for the reply, will give that a shot.

> 
> It seems we are going to revert/adapt 4cd13c21b2 , not defer more
> stuff to ksoftirqd.

Hmm, what if we use some other logic for deferring/non-deferring
like checking how many softirqs where serviced during process's
timeslice and decide if proceed with __do_softirq() or defer it
not to starve a task? Might that make sense?

-- 
Thanks,
             Dmitry