Date: Mon, 10 Dec 2018 17:13:55 -0700 From: Kevin Bowling <kevin.bowling@kev009.com> To: ian@freebsd.org Cc: John Baldwin <jhb@freebsd.org>, Warner Losh <imp@bsdimp.com>, mjguzik@gmail.com, svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers <src-committers@freebsd.org>, scottl@freebsd.org Subject: Re: svn commit: r341682 - head/sys/sys Message-ID: <CAK7dMtDgZWhFC7hF56jmdQF0KtfsrMM6=f1rn2SchP-4nXinpw@mail.gmail.com> In-Reply-To: <1544486233.1860.343.camel@freebsd.org> References: <201812071205.wB7C5BvA038350@repo.freebsd.org> <1544206201.1860.288.camel@freebsd.org> <CAGudoHGUqiByb37wcbhMD3eWmxFrX5=BKMiF3bk6Ptr2WWggAg@mail.gmail.com> <CAK7dMtBj26Fa-eS3WcKezjnrweDJ9RgZvBpbiRK-9S-A4Svb8Q@mail.gmail.com> <CANCZdfoXuKxuOztCU7Xoavak5b6KYB9bBQonssWUABgQabBBUQ@mail.gmail.com> <45f85061-2633-852c-3cc0-41f64d51e4f0@FreeBSD.org> <1544486233.1860.343.camel@freebsd.org>
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Right we are talking about a polyfill for systems that have 1-2 cores in practice. You're not going to crank high parallelism on these global locks in practice and the common lock may help performance due to cache residence for all we know. This is a lot of ballyhoo for a decision that should favor the reduction of complexity, clean KPIs, and overwhelming majority of machines that have 64b atomics, not scattering ifdefs in the code for niche performance. Regards, Kevin On Mon, Dec 10, 2018 at 4:57 PM Ian Lepore <ian@freebsd.org> wrote: > > On Mon, 2018-12-10 at 14:15 -0800, John Baldwin wrote: > > On 12/8/18 7:43 PM, Warner Losh wrote: > > > > > > > > > > > > On Sat, Dec 8, 2018, 8:36 PM Kevin Bowling <kevin.bowling@kev009.co > > > m <mailto:kevin.bowling@kev009.com> wrote: > > > > > > On Sat, Dec 8, 2018 at 12:09 AM Mateusz Guzik <mjguzik@gmail.co > > > m <mailto:mjguzik@gmail.com>> wrote: > > > > > > > > > > > Fully satisfying solution would be that all architectures get > > > 64-bit > > > > ops, even if in the worst case they end up taking a lock. > > > Then > > > > subsystems would not have to ifdef on anything. However, > > > there > > > > was some opposition to this proposal and I don't think this > > > is > > > > important enough to push. > > > > > > Mateusz, > > > > > > Who is opposing this particular polyfill solution? Scott Long > > > brought > > > up a situation in driver development where this would be useful > > > as > > > well. The polyfills lower the cognitive load and #ifdef soup > > > which > > > are the right call here regardless of performance on toy ports. > > > > > > > > > I don't recall seeing the opposition either. It would have to be a > > > global lock for all 64bit atomics.... but I think it would only be > > > 2 atomics on those architectures. > > It would have to be a spin lock, so in the case of unrl you would be > > trading > > an operation on one of N regular mutexes for a single spin lock that > > was > > also contested by other things. This would be pretty crappy. For > > drivers > > that aren't actually used on platforms without 32-bit atomics we can > > simply > > not build them in sys/modules/Makefile or not put them in > > GENERIC. For > > something in the core kernel like unrl I think we will have to do > > what > > Mateusz has done here. > > > > On a single-core system all you need to implement 64-bit atomics in the > kernel is to disable interrupts around using normal load/store > operations on the values. Do we have any platforms that are SMP but > don't have hardware primitives for 64-bit atomics? > > -- Ian
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