netdev - Re: [PATCH] r8169: Reduce looping in the interrupt handler.

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Message-ID: <20090901092012.GA3662@electric-eye.fr.zoreil.com>
Date:	Tue, 1 Sep 2009 11:20:12 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	David Dillow <dave@...dillows.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Michael Riepe <michael.riepe@...glemail.com>,
	Michael Buesch <mb@...sch.de>,
	Rui Santos <rsantos@...popie.com>,
	Michael B??ker <m.bueker@...lin.de>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] r8169: Reduce looping in the interrupt handler.

David Dillow <dave@...dillows.org> :
[...]
> I've not been able to reproduce my lockups under medium testing with
> Francois's patch applied, so I'm a little more comfortable with it.

Nice :o)

> At the same time, I'm worried that the timing just changed enough to
> make it harder to trigger, as was the case when I did the patch IIRC.

It is a legitimate concern.

> The kernel's interrupt handling changed in a manner that made it much
> easier to hit about that time. The testing I did in May pointed strongly
> at us failing to ACK an interrupt source, causing the MSI generation to
> stop, so I need to think about things some more.

It can be understood as us claiming to perform some work we didn't too.

In this regard, a "ack everything and perform no work loop in the irq
handler" design would require some work : it races with the - supposedly
fast, register read free - napi handler which does not check that
unprocessed events are acked.

As the current patch was provided with almost no explanation :
- the irq handler and the napi handler are allowed / assumed / expected
  to race
- the napi and irq handlers ack respectively their own events (IntrStatus).
  They do not ack their friend ones.
- everybody acks (IntrStatus) before the work is done
- napi irqs are disabled before napi is (tentatively) scheduled.
  napi irqs are only expected to be disabled most of the time the napi
  handler runs.
- the napi handler enables its irqs, tests new events and conditionaly
  schedules itself.

-- 
Ueimor
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