Date: Thu, 21 Feb 2013 23:28:27 +0100 From: Svatopluk Kraus <onwahe@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-current@freebsd.org Subject: Re: [patch] i386 pmap sysmaps_pcpu[] atomic access Message-ID: <CAFHCsPV9XfvhukSdSf0jm3x-m%2BZ9bneKQy3HM2Ru5Pa=2379SQ@mail.gmail.com> In-Reply-To: <201302211043.49906.jhb@freebsd.org> References: <CAFHCsPUVTM9jfrnzY72YsPszLWkg-UaJcycTR4xXcS%2BfPzS1Vg@mail.gmail.com> <201302201022.29294.jhb@freebsd.org> <CAFHCsPXEDiaKSkHNbycs%2B1DKkL7BcYJBk5eN%2BKpsLY8B%2B-u5gQ@mail.gmail.com> <201302211043.49906.jhb@freebsd.org>
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On Thu, Feb 21, 2013 at 4:43 PM, John Baldwin <jhb@freebsd.org> wrote: >> > curthread is a bit magic. :) If you perform a context switch during an >> > interrupt (which will change 'curthread') you also change your register state. >> > When you resume, the register state is also restored. This means that while >> > something like 'PCPU_GET(cpuid)' might be stale after you read it, 'curthread' >> > never is. However, it is true that actually reading curthread has to be >> > atomic. If your read of curthread looks like: >> > >> > mov <pcpu reg>, r0 >> > add <offset of pc_curthread>, r0 >> > ld r0, r1 >> > >> > Then that will indeed break. Alpha used a fixed register for 'pcpu_reg' >> > (as does ia64 IIRC). OTOH, you might also be able to depend on the fact that >> > pc_curthread is the first thing in 'struct pcpu' (and always will be, you could >> > add a CTASSERT to future-proof). In that case, you can remove the 'add' >> > instruction and instead just do: >> > >> > ld <pcpu reg>, r1 >> > >> > which is fine. >> >> Just for the record. There are three extra (coprocessor) registers per >> a core in arm11mpcore (armv6k). Unfortunately only one is Privileged. >> The other ones are respectively User Read only and User Read Write. >> For now, we are using the Privileged one to store pcpu pointer >> (curthread is correctly set during each context switch). Thus, using a >> coprocessor register means that reading of curthread is not a single >> instruction implementation now. After we figured out the curthread >> issue in SMP case, using a fixed (processor) register for pcpu is an >> option. Meanwhile, we disable interrupts before reading of curthread >> and enable them after. The same is done for other PCPU stuff. For now >> we have not stable system enough for profiling, however, when it will >> be, it would be interesting to learn how various implementations of >> curthread reading impact system performance. > > curthread is read _a lot_, so I would expect this to hurt. What we did on > Alpha was to use a fixed register for pcpu access, but we used the equivalent > of a coprocessor register to also store the value so we could set that fixed > register on entry to the kernel (userland was free to use the register for > its own purposes). Interesting solution. Thanks. Svata
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