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

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Message-ID: <20180111044456.GC11633@lerouge>
Date:   Thu, 11 Jan 2018 05:44:57 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dmitry Safonov <dima@...sta.com>,
        Eric Dumazet <edumazet@...gle.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>,
        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 Wed, Jan 10, 2018 at 08:19:49PM -0800, Linus Torvalds wrote:
> On Wed, Jan 10, 2018 at 7:22 PM, Frederic Weisbecker
> <frederic@...nel.org> wrote:
> >
> > Makes sense, but I think you need to keep the TASK_RUNNING check.
> 
> Yes, good point.
> 
> > So perhaps it should be:
> >
> > -       return tsk && (tsk->state == TASK_RUNNING);
> > +       return (tsk == current) && (tsk->state == TASK_RUNNING);
> 
> Looks good to me - definitely worth trying.
> 
> Maybe that weakens the thing so much that it doesn't actually help the
> UDP packet storm case?
> 
> And maybe it's not sufficient for the dvb issue.
> 
> But I think it's worth at least testing. Maybe it makes neither side
> entirely happy, but maybe it might be a good halfway point?

Yes I believe Dmitry is facing a different problem where he would rather
see ksoftirqd scheduled more often to handle the queue as a deferred batch
instead of having it served one by one on the tails of IRQ storms.
(Dmitry correct me if I misunderstood).

But your patch still seems to make sense for the case you described: when
ksoftirqd is voluntarily preempted off and the current IRQ could handle the
queue.