On Mon, 19 Oct 2020 12:33:12 +0200 Thomas Gleixner wrote: > On Sun, Oct 18 2020 at 10:19, Jakub Kicinski wrote: > > On Sun, 18 Oct 2020 10:20:41 +0200 Heiner Kallweit wrote: > >> >> Otherwise a non-solution could be to make IRQ_FORCED_THREADING > >> >> configurable. > >> > > >> > I have to say I do not understand why we want to defer to a thread the > >> > hard IRQ that we use in NAPI model. > >> > > >> Seems like the current forced threading comes with the big hammer and > >> thread-ifies all hard irq's. To avoid this all NAPI network drivers > >> would have to request the interrupt with IRQF_NO_THREAD. > > In a !RT kernel, forced threading (via commandline option) is mostly a > debug aid. It's pretty useful when something crashes in hard interrupt > context which usually takes the whole machine down. It's rather unlikely > to be used on production systems, and if so then the admin surely should > know what he's doing. > > > Right, it'd work for some drivers. Other drivers try to take spin locks > > in their IRQ handlers. > > I checked a few which do and some of these spinlocks just protect > register access and are not used for more complex serialization. So > these could be converted to raw spinlocks because their scope is short > and limited. But yes, you are right that this might be an issue in > general. > > > What gave me a pause was that we have a busy loop in napi_schedule_prep: > > > > bool napi_schedule_prep(struct napi_struct *n) > > { > > unsigned long val, new; > > > > do { > > val = READ_ONCE(n->state); > > if (unlikely(val & NAPIF_STATE_DISABLE)) > > return false; > > new = val | NAPIF_STATE_SCHED; > > > > /* Sets STATE_MISSED bit if STATE_SCHED was already set > > * This was suggested by Alexander Duyck, as compiler > > * emits better code than : > > * if (val & NAPIF_STATE_SCHED) > > * new |= NAPIF_STATE_MISSED; > > */ > > new |= (val & NAPIF_STATE_SCHED) / NAPIF_STATE_SCHED * > > NAPIF_STATE_MISSED; > > } while (cmpxchg(&n->state, val, new) != val); > > > > return !(val & NAPIF_STATE_SCHED); > > } > > > > > > Dunno how acceptable this is to run in an IRQ handler on RT.. > > In theory it's bad, but I don't think it's a big deal in reality.
Awesome, thanks for advice and clearing things up! Let me apply Heiner's IRQF_NO_THREAD patch, then.