linux-kernel - Re: regression with napi/softirq ?

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Message-ID: <CADVatmN6xNO1iMQ4ihsT5OqV2cuj2ajq+v00NrtUyOHkiKPo-Q@mail.gmail.com>
Date:   Thu, 18 Jul 2019 12:18:07 +0100
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "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 ?

Hi Eric,

On Thu, Jul 18, 2019 at 7:58 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 7/17/19 11:52 PM, Thomas Gleixner wrote:
> > Sudip,
> >
> > On Wed, 17 Jul 2019, Sudip Mukherjee wrote:
> >> On Wed, Jul 17, 2019 at 9:53 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >>> You can hack ksoftirq_running() to return always false to avoid this, but
> >>> that might cause application starvation and a huge packet buffer backlog
> >>> when the amount of incoming packets makes the CPU do nothing else than
> >>> softirq processing.
> >>
> >> I tried that now, it is better but still not as good as v3.8
> >> Now I am getting 375.9usec as the maximum time between raising the softirq
> >> and it starting to execute and packet drops still there.
> >>
> >> And just a thought, do you think there should be a CONFIG_ option for
> >> this feature of ksoftirqd_running() so that it can be disabled if needed
> >> by users like us?
> >
> > If at all then a sysctl to allow runtime control.
> >

<snip>

>
> 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. :(


-- 
Regards
Sudip