netdev - Excessive IPI interrupts with single RX queue devices and rps

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Message-ID: <CAGVrzcawQhyMXw+_0394OJ=UfiN1Lyxf+JThYPbWrJM6LSZ-WA@mail.gmail.com>
Date:	Thu, 19 Dec 2013 17:12:17 -0800
From:	Florian Fainelli <f.fainelli@...il.com>
To:	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>, therbert@...gle.com
Subject: Excessive IPI interrupts with single RX queue devices and rps_cpus
 mask set

Hello,

After configuring the rps_cpus mask to enable all CPUs for a single RX
queue network device, I noticed that I was getting one interrupt per
packet received, with the interrupt count being splitted between NAPI
interrupts and IPI SMP interrupts.

This behavior was observed on 3.8.13 as well as 3.3. The host was
receiving a Gigabit UDP stream and the receiver was not consuming the
SKBs. Whenever there was a listener consuming the SKBs received, the
number of IPI/sec quickly jumped to about 4 times the number without a
rps_cpus mask.

Shall we prevent the rps_cpu mask to be set at all for single RX queue
devices since this seems to causes excessive IPI interrupts?
-- 
Florian
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