netdev - Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT

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Date:   Fri, 14 May 2021 23:53:56 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alison Chaiken <achaiken@...ora.tech>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        netdev@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, sassmann@...hat.com,
        "David S. Miller" <davem@...emloft.net>, stable-rt@...r.kernel.org
Subject: Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT

On Fri, May 14 2021 at 12:44, Alison Chaiken wrote:
> On Fri, May 14, 2021 at 11:56 AM Jakub Kicinski <kuba@...nel.org> wrote:
>>
>> Another thing while I have your attention - ____napi_schedule() does
>> __raise_softirq_irqoff() which AFAIU does not wake the ksoftirq thread.
>> On non-RT we get occasional NOHZ warnings when drivers schedule napi
>> from process context, but on RT this is even more of a problem, right?
>> ksoftirqd won't run until something else actually wakes it up?
>
> By "NOHZ warnings," do you mean "NOHZ: local_softirq_pending"?    We see
> that message about once a week with 4.19.   Presumably any failure of
> ____napi_schedule() to wake ksoftirqd could only cause problems for the
> NET_RX softirq, so if the pending softirq is different, the cause lies
> elsewhere.

If you read the above carefully you might notice that this _IS_ about
____napi_schedule() being invoked from task context which raises NET_RX
and then results in pending 08! which is NET_RX.

Thanks,

        tglx