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    SubjectRe: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug
    * Jeremy Fitzhardinge (jeremy@goop.org) wrote:
    > Mathieu Desnoyers wrote:
    >> * H. Peter Anvin (hpa@zytor.com) wrote:
    >>
    >>> Mathieu Desnoyers wrote:
    >>>
    >>>> I can't argue about the benefit of using VM CPU pinning to manage
    >>>> resources because I don't use it myself, but I ran some tests out of
    >>>> curiosity to find if uncontended locks were that cheap, and it turns out
    >>>> they aren't. Here are the results :
    >>>> Xeon 2.0GHz
    >>>> Summary
    >>>> make -j1 kernel/ 33.94 +/- 0.07 34.91 +/- 0.27 2.8 %
    >>>> hackbench 50 2.99 +/- 0.01 3.74 +/- 0.01 25.1 %
    >>>> 1 CPU, replace smp lock prefixes with DS segment selector prefixes
    >>>> 1 CPU, noreplace-smp
    >>>>
    >>> For reference, could you also compare replace smp lock with NOPs?
    >>>
    >>> -hpa
    >>>
    >>
    >> Sure, here are the updated tables. Basically, they show no significant
    >> difference between the NOP and the DS segment selector prefix
    >> approaches.
    >>
    >
    > BTW, are you changing the initial prefix to DS too? Ie, are you doing a
    > nop->lock->ds transition, or ds->lock->ds?
    >
    > J

    Yeah, I thought about this case yesterday, good thing you ask.

    include/asm-x86/alternative.h defines LOCK_PREFIX as :

    #define LOCK_PREFIX \
    ".section .smp_locks,\"a\"\n" \
    _ASM_ALIGN "\n" \
    _ASM_PTR "661f\n" /* address */ \
    ".previous\n" \
    "661:\n\tlock; "

    So we have the locked instructions built into the kernel, not the nop'd
    one. Therefore, the only transition I am doing for my benchmarks is :

    lock->ds

    but I tried to switch back to SMP and it worked fine.

    Mathieu


    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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