Date: Fri, 06 Mar 2015 15:44:06 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: 'Andriy Gapon' <avg@freebsd.org> Subject: RFC: Simplfying hyperthreading distinctions Message-ID: <1640664.8z9mx3EOQs@ralph.baldwin.cx>
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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?
--
John Baldwin
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