netdev - Re: regression with napi/softirq ?

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Date:   Thu, 18 Jul 2019 14:40:49 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sudip Mukherjee <sudipm.mukherjee@...il.com>
cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: regression with napi/softirq ?

On Thu, 18 Jul 2019, Sudip Mukherjee wrote:
> On Thu, Jul 18, 2019 at 7:58 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
> > ksoftirqd might be spuriously scheduled from tx path, when
> > __qdisc_run() also reacts to need_resched().
> >
> > By raising NET_TX while we are processing NET_RX (say we send a TCP ACK packet
> > in response to incoming packet), we force __do_softirq() to perform
> > another loop, but before doing an other round, it will also check need_resched()
> > and eventually call wakeup_softirqd()
> >
> > I wonder if following patch makes any difference.
> >
> > diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> > index 11c03cf4aa74b44663c74e0e3284140b0c75d9c4..ab736e974396394ae6ba409868aaea56a50ad57b 100644
> > --- a/net/sched/sch_generic.c
> > +++ b/net/sched/sch_generic.c
> > @@ -377,6 +377,8 @@ void __qdisc_run(struct Qdisc *q)
> >         int packets;
> >
> >         while (qdisc_restart(q, &packets)) {
> > +               if (qdisc_is_empty(q))
> > +                       break;
> 
> unfortunately its v4.14.55 and qdisc_is_empty() is not yet introduced.
> And I can not backport 28cff537ef2e ("net: sched: add empty status
> flag for NOLOCK qdisc")
> also as TCQ_F_NOLOCK is there. :(

Then please run the test on a current kernel. Backports can be discussed
once the problem is understood.

Thanks,

	tglx