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@xxxxxxxxxx> > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > > Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > > --- > > 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?
