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Date:	Thu, 19 Dec 2013 17:30:54 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	netdev <netdev@...r.kernel.org>, therbert@...gle.com
Subject: Re: Excessive IPI interrupts with single RX queue devices and
 rps_cpus mask set

On Thu, 2013-12-19 at 17:12 -0800, Florian Fainelli wrote:
> 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?

Not sure what you mean.

This is the thing that highly depends on workload.

(Most workloads perform better without RPS/RFS)

This is why it is disabled by default.



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