linux-kernel - Re: [PATCH] net: Adjust softirq raising in __napi

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Message-ID: <4AE56217.3040708@imap.cc>
Date:	Mon, 26 Oct 2009 10:47:19 +0200
From:	Tilman Schmidt <tilman@...p.cc>
To:	Johannes Berg <johannes@...solutions.net>
CC:	Jarek Poplawski <jarkao2@...il.com>,
	David Miller <davem@...emloft.net>, hidave.darkstar@...il.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	linux-wireless@...r.kernel.org, linux-ppp@...r.kernel.org,
	netdev@...r.kernel.org, paulus@...ba.org,
	Michael Buesch <mb@...sch.de>,
	Oliver Hartkopp <oliver@...tkopp.net>
Subject: Re: [PATCH] net: Adjust softirq raising in __napi_schedule

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Am 26.10.2009 09:58 schrieb Johannes Berg:
> On Mon, 2009-10-26 at 07:54 +0000, Jarek Poplawski wrote:
> 
>>> No, I wrote that I didn't know. I suppose now that I looked at it I do
>>> know, and only disabling preemption is required.
>> But netif_rx has preemption disabled most of the time (by hardirqs
>> disabling). So maybe disabling preemption isn't the main reason here
>> either?
> 
> Not for netpoll though, which may or may not be relevant (if I were to
> venture a guess I'd say it isn't and it disables preemption to be able
> to do the softirq thing)
> 
> However, I lost track now of why we're discussing this.

The starting point were several reports of the kernel message:

NOHZ: local_softirq_pending 08

Originally most if not all of them came from wireless networking,
but I muddied the waters by adding to the mix a case involving ISDN.
You stated that all the solutions proposed so far were wrong, so
we're naturally turning to you for guidance on what the right
solution might be.

> Basically it boils down to using netif_rx() when in (soft)irq, and
> netif_rx_ni() when in process context. That could just be an
> optimisation, but it's a very valid one.

Hmmm. That seems to contradict your earlier statement to me that
simply replacing a call to netif_rx() by one to netif_rx_ni()
when not in interrupt context isn't a valid solution either.
What am I missing?

Thanks,
Tilman

- --
Tilman Schmidt                    E-Mail: tilman@...p.cc
Bonn, Germany
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