Date: Fri, 06 Mar 2015 12:59:04 -0800 From: Rui Paulo <rpaulo@me.com> To: John Baldwin <jhb@FreeBSD.org> Cc: Andriy Gapon <avg@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: RFC: Simplfying hyperthreading distinctions Message-ID: <145BFBD8-7092-42F3-BCF3-5CBB78D15893@me.com> In-Reply-To: <1640664.8z9mx3EOQs@ralph.baldwin.cx> References: <1640664.8z9mx3EOQs@ralph.baldwin.cx>
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> On 6 Mar 2015, at 12:44, John Baldwin <jhb@FreeBSD.org> wrote:
>=20
> Currently we go out of our way a bit to distinguish Pentium4-era=20
> hyperthreading from more recent ("modern") hyperthreading. I suspect =
that=20
> this distinction probably results in confusion more than anything =
else. =20
> Intel's documentation does not make near as broad a distinction as far =
as I=20
> can tell. Both types of SMT are called hyperthreading in the SDM for =
example. =20
> However, we have the astonishing behavior that=20
> 'machdep.hyperthreading_allowed' only affects "old" hyperthreads, but =
not=20
> "new" ones. We also try to be overly cute in our dmesg output by =
using HTT=20
> for "old" hyperthreading, and SMT for "new" hyperthreading.
Yes, this is annoying.
> I propose the=20
> following changes to simplify things a bit:
>=20
> 1) Call both "old" and "new" hyperthreading HTT in dmesg.
Yes.
> 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.
I think that's ok given 3).
> 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.
Sounds good.
Thanks,
--
Rui Paulo
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