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

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Message-ID: <20210514133815.6a698aeb@kicinski-fedora-PC1C0HJN>
Date:   Fri, 14 May 2021 13:38:15 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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, 14 May 2021 22:16:10 +0200 Thomas Gleixner wrote:
> On Fri, May 14 2021 at 11:56, Jakub Kicinski wrote:
> > On Thu, 13 May 2021 00:28:02 +0200 Thomas Gleixner wrote:  
> >> 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.

Sounds reasonable. I'll send a patch with a new helper and convert 
an example driver I'm sure falls into the "napi_schedule(); return;"
category. I just want to make sure "the right thing to do" is
accessible for people writing new drivers.