Re: Excessive interrupt time on Raspberry Pi

[Jared McNeill <jmcneill%invisible.ca@localhost>]

Nick tells me the microframe scheduler has to run on the host if at least 
one device is connected and since you can't remove the built-in hub or 
ethernet from rpi, you're screwed. Raspbian kernel uses FIQ to help 
here..


On Sat, 26 May 2018, Jason Thorpe wrote:

> So, my Raspberry Pi is sitting completely idle other than an ssh session (over USB-connected-WiFi) and an instance of top(1) running:
>
> load averages:  0.02,  0.02,  0.00;               up 0+08:19:41        06:10:44
> 22 processes: 21 sleeping, 1 on CPU
> CPU states:  0.0% user,  0.0% nice,  0.0% system,  4.8% interrupt, 95.2% idle
> Memory: 191M Act, 584K Inact, 12M Wired, 13M Exec, 170M File, 175M Free
> Swap: 128M Total, 128M Free
>
> That interrupt time is **insane**.
>
> Turns out it’s the USB:
>
> bcm2835 pic timer3       3115038   99
> bcm2835 pic Mailbox         1039    0
> bcm2835 pic uart            7797    0
> bcm2835 pic emmc           70272    2
> bcm2835 pic dma0           18396    0
> bcm2835 pic usb        251638013 8075
> bcm2835 pic Doorbell0          4    0
> Total                  254850559 8178
>
> If I unplug my urtwn WiFi and reboot, the situation isn’t any better; with nothing plugged into the USB ports, I’m still getting ~8K interrupts per second from USB:
>
> interrupt               total rate
> bcm2835 pic timer3       9795  104
> bcm2835 pic Mailbox         4    0
> bcm2835 pic uart         3430   36
> bcm2835 pic emmc        38303  407
> bcm2835 pic dma0        12589  133
> bcm2835 pic usb        770580 8197
> bcm2835 pic Doorbell0       4    0
> Total                  834705 8879
>
> Do other platforms have use the DesignWare USB controller have a similar problem?
>
> -- thorpej