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    Subject[patch, 2.6.10-rc3] safe_hlt() & NMIs

    * Lee Revell <rlrevell@joe-job.com> wrote:

    > On Sun, 2004-12-12 at 13:15 +0100, Andrea Arcangeli wrote:
    > > Overall this is a very minor issue (unless HZ is 0), it would only
    > > introduce a 1/HZ latency to the irq that get posted while the nmi
    > > handler is running, and the nmi handlers never runs in production.
    >
    > Ingo, couldn't this account for some of the inexplicable outliers some
    > people were seeing in latency tests?

    indeed, there could be a connection, and it's certainly a fun race. The
    proper fix is Manfred's suggestion: check whether the EIP is a kernel
    text address, and if yes, whether it's a HLT instruction - and if yes
    then increase EIP by 1. I've included the fix in the -33-02 -RT patch.
    Andrew, Linus: upstream fix is below - i think it's post-2.6.10 stuff.
    Tested it on SMP and UP x86, using both the IO-APIC and the local-APIC
    based NMI watchdog.

    i think x64 needs a similar fix as well.

    Ingo

    --- linux/arch/i386/kernel/traps.c.orig
    +++ linux/arch/i386/kernel/traps.c
    @@ -670,6 +670,17 @@ fastcall void do_nmi(struct pt_regs * re

    cpu = smp_processor_id();

    + /*
    + * Fix up obscure CPU behavior: if we interrupt safe_hlt() via
    + * the NMI then we might miss a reschedule if an interrupt is
    + * posted to the CPU and executes before the HLT instruction.
    + *
    + * We check whether the EIP is kernelspace, and if yes, whether
    + * the instruction is HLT:
    + */
    + if (__kernel_text_address(regs->eip) && *(char *)regs->eip == 0xf4)
    + regs->eip++;
    +
    #ifdef CONFIG_HOTPLUG_CPU
    if (!cpu_online(cpu)) {
    nmi_exit();
    -
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