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

[Alison Chaiken <achaiken@aurora.tech>]

On Fri, May 14, 2021 at 11:56 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Thu, 13 May 2021 00:28:02 +0200 Thomas Gleixner wrote:
> > On Wed, May 12 2021 at 23:43, Sebastian Andrzej Siewior wrote:
> > > __napi_schedule_irqoff() is an optimized version of __napi_schedule()
> > > which can be used where it is known that interrupts are disabled,
> > > e.g. in interrupt-handlers, spin_lock_irq() sections or hrtimer
> > > callbacks.
> > >
> > > On PREEMPT_RT enabled kernels this assumptions is not true. Force-
> > > threaded interrupt handlers and spinlocks are not disabling interrupts
> > > and the NAPI hrtimer callback is forced into softirq context which runs
> > > with interrupts enabled as well.
> > >
> > > Chasing all usage sites of __napi_schedule_irqoff() is a whack-a-mole
> > > game so make __napi_schedule_irqoff() invoke __napi_schedule() for
> > > PREEMPT_RT kernels.
> > >
> > > The callers of ____napi_schedule() in the networking core have been
> > > audited and are correct on PREEMPT_RT kernels as well.
> > >
> > > Reported-by: Juri Lelli <juri.lelli@redhat.com>
> > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> >
> > Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> >
> > > ---
> > > 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.
>
> 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?
>
>
> 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.

-- Alison Chaiken
   Aurora Innovation