RFC: Simplfying hyperthreading distinctions

[Slawa Olhovchenkov]

On Fri, Mar 06, 2015 at 04:17:37PM -0500, John Baldwin wrote:

> 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).

Can you explain interrupt handling some more?
How routing real interrupt? Can be real interrupt routing to specific
core?
Is real interrupt and `cpuset -x irq` is same (I see interrup from
chelsio on cpu other then pinned)?