netdev - Re: Remove __napi_schedule

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Date:   Mon, 19 Oct 2020 10:55:16 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Remove __napi_schedule_irqoff?

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.