The machdep.hyperthreading_allowed & ULE weirdness in 7.1

[Maxim Sobolev]

Robert Watson wrote:
> 
> On Sun, 22 Feb 2009, Maxim Sobolev wrote:
> 
>> Hi Jeff,
>>
>> I have a single-CPU system with P4 HTT-enabled processor 
>> (7.1-RELEASE-p3), kernel compiled with SCHED_ULE.
> 
> This is because machdep.hlt_logical_cpus doesn't do what you think it 
> does. It causes HTT cores to invoke the hlt instruction in their idle 

Hmm, as the subject says I am actually talking about flipping 
machdep.hyperthreading_allowed, not machdep.hlt_logical_cpus sysctl 
here. I provided current value of the latter only to simplify diagnosis 
and had not changed it from the default value. AFAIK, the 
machdep.hyperthreading_allowed is designed to prevent logical cores from 
running any code, works just fine with 6.x/SCHED_4BSD.

Below are screenshots from the dual-core 6.2 system with 4BSD. As you 
can easily see, after flipping machdep.hyperthreading_allowed only cores 
0 and 2 run actual code, so that it's definitely regression of ULE/7.x:

machdep.hyperthreading_allowed=1:
  top: http://sobomax.sippysoft.com/~sobomax/ScreenShot462.png

machdep.hyperthreading_allowed=0:
  top: http://sobomax.sippysoft.com/~sobomax/ScreenShot463.png

IMHO, at the very least, if SCHED_ULE doesn't support 
machdep.hyperthreading_allowed properly, then when that scheduler is 
selected the sysctl should return ENOTSUP or something like this.

-Maxim