Date: Fri, 06 Mar 2015 18:14:01 -0800 From: Nathan Whitehorn <nwhitehorn@freebsd.org> To: John Baldwin <jhb@freebsd.org>, freebsd-arch@freebsd.org Cc: 'Andriy Gapon' <avg@freebsd.org> Subject: Re: RFC: Simplfying hyperthreading distinctions Message-ID: <54FA5EE9.4090305@freebsd.org> In-Reply-To: <1640664.8z9mx3EOQs@ralph.baldwin.cx> References: <1640664.8z9mx3EOQs@ralph.baldwin.cx>
next in thread | previous in thread | raw e-mail | index | archive | help
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 fine with whatever naming, but if we're making new sysctls,
especially for the cpuset case, is there a reason to hide the behavior
under machdep? We support at least three non-x86 CPUs with SMT (POWER8,
Cell, and POWER5) and the relevant scheduling logic should be MI. At
least POWER8 supports 8 threads per core, so you might also want more
granularity than just "on" or "off".
-Nathan
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?54FA5EE9.4090305>