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Date: Sun, 18 Oct 2020 10:19:47 -0700 From: Jakub Kicinski <kuba@...nel.org> To: Heiner Kallweit <hkallweit1@...il.com> Cc: Eric Dumazet <edumazet@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, Eric Dumazet <eric.dumazet@...il.com>, David Miller <davem@...emloft.net>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: Remove __napi_schedule_irqoff? 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. Right, it'd work for some drivers. Other drivers try to take spin locks in their IRQ handlers. 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..
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