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>
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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
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