Date: Fri, 13 Mar 2015 13:58:37 -0400 From: John Baldwin <jhb@freebsd.org> To: Nathan Whitehorn <nwhitehorn@freebsd.org> Cc: 'Andriy Gapon' <avg@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: RFC: Simplfying hyperthreading distinctions Message-ID: <2058936.CdnVTM8mMb@ralph.baldwin.cx> In-Reply-To: <54FA5EE9.4090305@freebsd.org> References: <1640664.8z9mx3EOQs@ralph.baldwin.cx> <54FA5EE9.4090305@freebsd.org>
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On Friday, March 06, 2015 06:14:01 PM Nathan Whitehorn 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 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". 3) would involve something new, yes, but my immediate concern is more 1) and 2) to make x86 more consistent. I would certainly be fine with having an MI name for 3). When I've prototyped it before I did it in an MD SYSINIT, but I can perhaps make use of the topology flags to do this in MI code instead. -- John Baldwin
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