[LTP] hrimer-prio test

[Joerg Vehlow]

Hi Thomas,

I guess no one on the ltp mailing list can help here, maybe since you
are maintainer of the hrtimer subsystem, maybe you can help.

Thanks,
Joerg

Am 2/11/2022 um 7:56 AM schrieb Joerg Vehlow:
> Hi,
> 
> I was looking into fails with the test ltp test hrtimer-prio.
> 
> For reference here is the test description:
> ----
> Test the latency of hrtimers under rt load.	
> The busy_threads should run at a priority higher than the system
> softirq_hrtimer, but lower than the timer_thread.  The timer_thread
> measure the time it takes to return from a nanosleep call.  If the
> lower priority threads can increase the latency of the higher
> priority thread, it is considered a failure.
> ----
> 
> The test fails, repeatedly calls clock_nanosleep(10ms) and if the
> latency of one of these nanosleeps is longer than 10us, the test fails.
> The latency is measured using clock_gettime(CLOCK_MONOTONIC) In my test
> on x86_64 hardware, the test sometimes fails, but mostly passes. On an
> arm board it almost fails, sometimes with a minimal latency of > 30ms.
> 
> I have no knowledge of the timer subsystem, so I can't even tell if the
> test is (still) valid. The test is in ltp since 2007, but wasn't enabled
> until last year. So maybe the test is invalid and the expected latency
> is not even guaranteed anymore or does it require a specific kernel
> config or hardware (e.g. a high precision hardware timer)?
> 
> There is a kernel selftest for something similar (nsleep-lat). It only
> expects the average latency to be less than 40ms, but that has another
> difference, because it uses TIMER_ABSTIME instead of a relative sleep time.
> 
> Joerg
>