Date: Tue, 4 Jun 2013 11:07:05 -0700 From: Adrian Chadd <adrian@freebsd.org> To: Warner Losh <imp@bsdimp.com> Cc: FreeBSD-arch <freebsd-arch@freebsd.org>, "freebsd-mips@FreeBSD.org" <freebsd-mips@freebsd.org>, Ed Schouten <ed@80386.nl> Subject: Re: Kernelspace C11 atomics for MIPS Message-ID: <CAJ-VmonUs91yFjtM79RrRBE=WV-=XmP2ebiNi1M1WewqoO_oig@mail.gmail.com> In-Reply-To: <232DBBD8-3F32-4D42-85AB-AC5647EEA768@bsdimp.com> References: <CAJOYFBD502MYbkVR2hnVDTYWOvOUr15=OPyjotNvv%2BZ09vQ1OQ@mail.gmail.com> <D02AF210-5129-40AB-9481-3F0A44575E98@bsdimp.com> <CAJ-Vmo=vNbT9majPCZ8ugzPsNzh46DTD4mMDX-cuxx9Og91ptw@mail.gmail.com> <CACVs6=_X5vOfR%2BQOgvz6P-j3jUoNoK9hCFvz80fGRL3-PgBf5g@mail.gmail.com> <CAJ-Vmo=5%2BmWk4EWBuTdpF6vKx-%2BK=g1euJvZuRDF%2BvFkJNZZ4A@mail.gmail.com> <CAD44qMUUQ6_FDiTknK1rcW=rs7Mz96X35QgnRxH%2Br5=ZqGRbAw@mail.gmail.com> <CAD44qMV2indeyD-OsJsyR_yp-HowJOrKG6Znx3zHkSKzt7xQzA@mail.gmail.com> <232DBBD8-3F32-4D42-85AB-AC5647EEA768@bsdimp.com>
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Hi, It survived an overnight thrashing on my AR7161 (mips24k.) Adrian On 3 June 2013 22:07, Warner Losh <imp@bsdimp.com> wrote: > Please find attached a simple patch that I'd like all MIPS users to try. > > Warner > > > > > On Jun 3, 2013, at 10:15 PM, Patrick Kelsey wrote: > >> On Tue, Jun 4, 2013 at 12:08 AM, Patrick Kelsey <kelsey@ieee.org> wrote: >>> On Mon, Jun 3, 2013 at 11:57 PM, Adrian Chadd <adrian@freebsd.org> wrote: >>>> On 3 June 2013 20:55, Juli Mallett <jmallett@freebsd.org> wrote: >>>> >>>>> To drain the pipeline on certain deficient (and mostly older) CPUs by way of >>>>> guesswork and a little vague magic. Most CPUs we support, I would guess, do >>>>> not need this, and it continues to exist solely for hysterical reasons. >>>> >>>> How can I turn it off for my compiles? >>>> >>>>> I've certainly gotten rid of them and some other cargo cult synchronization >>>>> on Octeon for testing and had it survive under considerable load, and >>>>> occasionally with some slight speedups (for some more commonly-used or >>>>> slower things than Just a Bunch Of NOPs.) >>>> >>>> Right. Well, since it's happening on every inlined lock, it's a bit silly. >>>> >>>>> The trouble is that proving they aren't necessary requires being rigorous >>>>> and careful in understanding documentation and errata, and FUD about their >>>>> possible necessity is somewhat-intimidating. It's not an easy kind of >>>>> corruption/unreliability/etc., to prove the lack of empirically. >>>> >>>> I've checked the diassembly from gcc-4.mumble on linux; it doesn't >>>> include NOPs like this as far as I can tell. >>>> >>> >>> The sync + 8 nops is coming from the definition of mips_sync() in >>> sys/mips/include/atomic.h. >>> >>> I agree with Juli that it appears to be a manual pipeline-flush >>> holdover from earlier days - I'm guessing there's 8 nops because the >>> R4000/4400 had both the sync instruction and an 8-stage pipeline. I'm >>> further guessing this was an attempt at providing stronger ordering >>> semantics than the sync instruction itself for the following >>> mb()/wmb()/rmb() definitions that use it, as the sync instruction >>> definition doesn't restrict execution of the before/after loads/stores >>> with respect to the sync instruction itself. >> >> Forgot to emphasize that this particular bit of old-school >> nop-counting is either pointless or a latent hazard - 8 does not cover >> the deepest MIPS pipeline around, then there's superscalar issue to >> consider - so I think it's either unnecessary or insufficient. So >> far, that's all criticism and no solution :/ >> _______________________________________________ >> freebsd-arch@freebsd.org mailing list >> http://lists.freebsd.org/mailman/listinfo/freebsd-arch >> To unsubscribe, send any mail to "freebsd-arch-unsubscribe@freebsd.org" > >
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