Date: Wed, 18 May 2011 13:04:13 -0400 From: Attilio Rao <attilio@freebsd.org> To: Garrett Cooper <yanegomi@gmail.com> Cc: "freebsd-current@freebsd.org" <freebsd-current@freebsd.org>, Andriy Gapon <avg@freebsd.org>, "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org> Subject: Re: [rfc] remove hlt_cpus et al sysctls and related code Message-ID: <BANLkTikMZ_xs4WCJVJG4oHe3rOKU8rqfVw@mail.gmail.com> In-Reply-To: <BBCD9D8C-FCAF-4DE3-9F66-4B65AAABE67B@gmail.com> References: <4DD3F662.9040603@FreeBSD.org> <BANLkTikOTe9ut3GFx0bhOernKandRGLhPg@mail.gmail.com> <BANLkTinVGrLoAOS_ZQ1YVB_Fw1cvf5kHyA@mail.gmail.com> <BBCD9D8C-FCAF-4DE3-9F66-4B65AAABE67B@gmail.com>
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2011/5/18 Garrett Cooper <yanegomi@gmail.com>: > On May 18, 2011, at 9:49 AM, Attilio Rao <attilio@freebsd.org> wrote: > >> 2011/5/18 Garrett Cooper <yanegomi@gmail.com>: >>> On Wed, May 18, 2011 at 9:40 AM, Andriy Gapon <avg@freebsd.org> wrote: >>>> >>>> I think that it is a well known fact that currently we do not have any= support for >>>> dynamically offlining processors. =C2=A0Yet, we have some code that lo= oks like it does >>>> provide that support and even provides a user interface to supposedly = do that. >>>> >>>> What we don't currently do specifically: >>>> - rebinding interrupts away from an offlined processor >>>> - updating relevant cpu sets and masks >>>> - protecting the above for concurrent access >>>> - moving threads away from an offlined processor >>>> - notifying potentially interested parties >>>> - maybe more... >>>> >>>> The code has been in this shape for a long while and I would dare to s= ay that it >>>> never really worked, not in "production ready" sense anyway. >>>> An example of troubles caused by using that code can be found e.g. in = the >>>> followups to the following PR: >>>> http://www.freebsd.org/cgi/query-pr.cgi?pr=3D145385 >>>> And also discussed here: >>>> http://thread.gmane.org/gmane.os.freebsd.stable/74462/focus=3D74510 >>>> >>>> I think that there already have been a proposal to remove the systcls = and the >>>> code. =C2=A0I would like to re-submit that proposal. >>>> Removing that code would: >>>> 1) prevent users from hurting themselves by executing broken code >>>> 2) potentially make things easier for largeSMP project >>>> >>>> Once we grow correct code for offlining CPUs, then we could re-introdu= ce the >>>> sysctls without any problems. >>>> While the offlining code doesn't seem terribly hard to develop, it's a= big piece >>>> of work and requires time and effort. >>> >>> =C2=A0 =C2=A0What would be nice too (even though it might not be possib= le) is >>> to make this more MI than it is today (i.e. sysctls that work for >>> amd64, sparc64, etc), but that might be a pipe dream. >>> Thanks! >>> -Garrett >> >> That is actually the purpose. =C2=A0We should have a real online/offline >> system for hotplugging CPUs, not only tied to x86 hyperthreading. >> The htt specific parts are mostly hacks that don't take into account >> all the necessary handover for it. >> >> Andryi, I'll look into the patch asap, but I'm in favor of this change f= or sure. > > =C2=A0 =C2=A0We use this internally at work still with a software config = that uses 4BSD so as long as there is an equivalent tunable, that's good en= ough for us moving forward. Tunables are pretty much acceptable for this case. What is really broken is the on-the-fly ability to mark CPUs active/inactive and subsequent handovers. I thought Andriy attached a patch to the tree, but it doesn't seem so... anyway, yes, I think that adding tunables for this is very reasonable and not as dangerous as the current mechanism. Attilio --=20 Peace can only be achieved by understanding - A. Einstein
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