netdev - Re: Remove __napi_schedule

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Message-ID: <20201018101947.419802df@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
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..