USB: OHCI: high softirq load

[johan@kernel.org (Johan Hovold)]

[ +CC: linux-usb ]

On Mon, Jan 16, 2017 at 12:14:03PM +0100, Boris Brezillon wrote:
> On Mon, 16 Jan 2017 11:54:23 +0100
> Antoine Aubert <a.aubert@overkiz.com> wrote:
> 
> > Also, I made a big misunderstanding
> > 
> > With EHCI + OHCI = high level of softirq (USB2.0)
> 
> Well, the number of irqs and softirqs are likely to be related (you
> usually trigger a softirq after you received an hardirq).
> 
> > 
> > OHCI only = normal level
> 
> What about EHCI only? And what happens if you only plug 1 device?
> Please share the content of /proc/interrupts (and everything you think
> is relevant) for each of these cases.
> 
> > 
> > Le 16/01/2017 ? 11:31, Antoine Aubert a ?crit :
> > > Thx for your answer Boris
> > >
> > > Le 16/01/2017 ? 10:02, Boris Brezillon a ?crit :  
> > >> Hi Antoine,
> > >>
> > >> On Mon, 16 Jan 2017 08:45:58 +0100
> > >> Antoine Aubert <a.aubert@overkiz.com> wrote:
> > >>  
> > >>> Hi,
> > >>>
> > >>> Im working on a AT91SAM9G25cu board
> > >>> (arch/arm/boot/dts/at91-kizboxmini.dts). We use linux-4.1.31, and when
> > >>> OHCI is enabled, I got some wired effects.  
> > >> Can you test on a more recent kernel (4.9 or 4.10-rc4)?  
> > > I'll give a try, just need little time ;)  
> > >>> eg with 3 FTDI pluged, interrupts: more than 3.5k/s, cpu softirq > 24%,
> > >>> loadavg > 0.5  
> > >> Can you check which interrupt is triggered (cat /proc/interrupts),  
> > > cat /proc/interrupts
> > >            CPU0      
> > >  16:       2286  atmel-aic   1 Level     pmc, at91_tick, at91_rtc, ttyS0
> > >  17:          0       PMC  17 Level     main_rc_osc
> > >  18:          0       PMC   0 Level     main_osc
> > >  19:          0       PMC  16 Level     mainck
> > >  20:          0       PMC   1 Level     clk-plla
> > >  21:          0       PMC   6 Level     clk-utmi
> > >  22:          0       PMC   3 Level     clk-master
> > >  23:     945527  atmel-aic  17 Level     tc_clkevt
> > >  24:      21815  atmel-aic  20 Level     at_hdmac
> > >  25:          0  atmel-aic  21 Level     at_hdmac
> > >  30:     120299  atmel-aic  24 Level     eth0
> > >  31:   22783651  atmel-aic  22 Level     ehci_hcd:usb1, ohci_hcd:usb2
> > >  99:          0      GPIO  16 Edge      PB_RST
> > > 100:          0      GPIO  17 Edge      PB_PROG
> > > Err:          0

Note that the ftdi driver uses a low-latency setting by default which
implies that the device sends a status update every millisecond. Hence,
the 1k interrupts per second (per device) while the port is open is
expected.

You can disable the low-latency behaviour using setserial:

	setserial /dev/ttyUSB0 ^low_latency

and see the number of interrupts drop to 1/16th. This can then be
reduced further by changing the latency_timer from its
(non-low-latency) default of 16 ms, for example:

	echo 64 >/sys/bus/usb-serial/devices/ttyUSB0/latency_timer

Johan