Date: Fri, 06 Mar 2015 16:17:37 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: Harrison Grundy <harrison.grundy@astrodoggroup.com> Subject: Re: RFC: Simplfying hyperthreading distinctions Message-ID: <1526311.uylCbgv5VB@ralph.baldwin.cx> In-Reply-To: <54FA1180.3080605@astrodoggroup.com> References: <1640664.8z9mx3EOQs@ralph.baldwin.cx> <54FA1180.3080605@astrodoggroup.com>
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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
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