[PATCH 2/2] selftests/powerpc: Calculate spin time in tm-unavailable

[Michael Ellerman]

Cyril Bur <cyrilbur at gmail.com> writes:

> On Tue, 2017-11-21 at 11:31 -0200, Gustavo Romero wrote:
>> Hi Cyril,
>> 
>> On 21-11-2017 05:17, Cyril Bur wrote:
>> > Currently the tm-unavailable test spins for a fixed amount of time in
>> > an attempt to ensure the FPU/VMX/VSX facilities are off. This value was
>> > experimentally tested to be long enough.
>> > 
>> > Problems may arise if kernel heuristics were to change. This patch
>> > should future proof this test.
>> 
>> I've tried it on a VM running on '4.14.0-rc7' and apparently it gets stuck
>> pretty slow on calibration, since it ran ~7m without finding the correct value
>> (before it would take about 3m), like:
>> 
>> $ time ./tm-unavailable
>> Testing required spin time required for facility unavailable...
>> 	Trying 0x18000000...
>> 	Trying 0x19000000...
>> 	Trying 0x1a000000...
>> ...
>> 	Trying 0xfd000000... ^C
>> 
>> real	7m15.304s
>> user	7m15.291s
>> sys	0m0.004s
>> 
>
> Interesting! I didn't test in a VM. I guess hypervisor switching
> completely changes the heuristic. Ok I'll have to rethink.
>
> Maybe the increase should be a multiplier to get to a good state more
> quickly.

Yeah this sucks in a VM:

# /home/michael/tm-unavailable 
Testing required spin time required for facility unavailable...
	Trying 0x18000000... 
	Trying 0x19000000... 
...
	Trying 0x110000000...  

etc.

I got sick of waiting for it, but it's causing my selftests job to time
out so it must be taking > ~1 hour.

cheers