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

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Date:   Fri, 14 May 2021 22:16:10 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jakub Kicinski <kuba@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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 11:56, Jakub Kicinski wrote:
> On Thu, 13 May 2021 00:28:02 +0200 Thomas Gleixner wrote:
>> > ---
>> > Alternatively __napi_schedule_irqoff() could be #ifdef'ed out on RT and
>> > an inline provided which invokes __napi_schedule().
>> >
>> > This was not chosen as it creates #ifdeffery all over the place and with
>> > the proposed solution the code reflects the documentation consistently
>> > and in one obvious place.  
>> 
>> Blame me for that decision.
>> 
>> No matter which variant we end up with, this needs to go into all stable
>> RT kernels ASAP.
>
> Mumble mumble. I thought we concluded that drivers used on RT can be
> fixed, we've already done it for a couple drivers (by which I mean two).
> If all the IRQ handler is doing is scheduling NAPI (which it is for
> modern NICs) - IRQF_NO_THREAD seems like the right option.

Yes. That works, but there are a bunch which do more than that IIRC.

> Is there any driver you care about that we can convert to using
> IRQF_NO_THREAD so we can have new drivers to "do the right thing"
> while the old ones depend on this workaround for now?

The start of this thread was about i40e_msix_clean_rings() which
probably falls under the IRQF_NO_THREAD category, but I'm sure that
there are others. So I chose the safe way for RT for now.

> 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?

Correct. I sent a patch for the r8152 usb network driver today which
suffers from that problem. :)

As I said there, we want a (debug/lockdep) check in __napi_schedule()
whether soft interrupts are disabled, but let me have a look whether
that check might make more sense directly in __raise_softirq_irqoff().

Thanks,

        tglx