RFC: Simplfying hyperthreading distinctions

[John Baldwin]

On Friday, March 06, 2015 12:43:44 PM Harrison Grundy wrote:
> On 03/06/15 12:44, John Baldwin wrote:
> > Currently we go out of our way a bit to distinguish Pentium4-era
> > hyperthreading from more recent ("modern") hyperthreading.  I
> > suspect that this distinction probably results in confusion more
> > than anything else. Intel's documentation does not make near as
> > broad a distinction as far as I can tell.  Both types of SMT are
> > called hyperthreading in the SDM for example. However, we have the
> > astonishing behavior that 'machdep.hyperthreading_allowed' only
> > affects "old" hyperthreads, but not "new" ones.  We also try to be
> > overly cute in our dmesg output by using HTT for "old"
> > hyperthreading, and SMT for "new" hyperthreading.  I propose the
> > following changes to simplify things a bit:
> > 
> > 1) Call both "old" and "new" hyperthreading HTT in dmesg.
> > 
> > 2) Change machdep.hyperthreading_allowed to apply to both new and
> > old HTT. However, doing this means a POLA violation in that we
> > would now disable modern HTT by default.  Balanced against
> > re-enabling "old" HTT by default on an increasingly-shrinking pool
> > of old hardware, I think the better approach here would be to also
> > change the default to allow HTT.
> > 
> > 3) Possibly add a different knob (or change the behavior of
> > machdep.hyperthreading_allowed) to still bring up hyperthreads, but
> > leave them out of the default cpuset (set 1).  This would allow
> > those threads to be re-enabled dynamically at runtime by adjusting
> > the mask on set 1. The original htt settings back when
> > 'hyperthreading_allowed' was introduced actually permitted this via
> > by adjusting 'machdep.hlt_cpus' at runtime.
> > 
> > What do people think?
> 
> I'm not sure of how interrupt handling works as it relates to HTT, but
> wouldn't using cpuset potentially leave them active for interrupt
> handling?
> 
> Other than that question, this all makes sense to me.

Interrupt handling works differently.  Per my commit a few minutes ago, we do 
not send interrupts to hyperthreads by default (either old or new).  However, 
ithreads that are not explicitly bound to a specific CPU will "float" among 
all the CPUs in set 1 so 3) would affect that.  Eventually I want to use a 
separate cpuset for interrupts that ithreads inherit from (rather than 
belonging to set 1).

-- 
John Baldwin