[ag-automation] echo 1 > /sys/kernel/debug/tracing/latency_hist/enable/preemptirqsoff causes crash

[Carsten Emde]

Matthias,

> [..]
> This loop runs just fine for a couple of hours. Then I start to see
> periodic latencies. The IRQs show latencies of +/- 20 usecs. But the
> return of the read()/select() show rising latencies, which eventually
> reach more than 2000 usecs. At this time, everything is then of course a
> big mess and I see in
> /sys/kernel/debug/tracing/latency_hist/timerandwakeup large latencies.
> Funny enough they seem to come from swapper.
Did you forget to disable sleep states?

This is best done by writing "0" to /dev/cpu_dma_latency such as

int fd = open("/dev/cpu_dma_latency", O_WRONLY);
if (fd >= 0)
   write(fd, "0", 1);
/* do not close fd */

Thanks,
	-Carsten.