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    Subject[PATCH REGRESSION FIX] x86 idle: restore mwait_idle()
    Date
    From: Len Brown <len.brown@intel.com>

    In Linux-3.9 we removed the mwait_idle() loop:
    'x86 idle: remove mwait_idle() and "idle=mwait" cmdline param'
    (69fb3676df3329a7142803bb3502fa59dc0db2e3)

    The reasoning was that modern machines should be sufficiently
    happy during the boot process using the default_idle() HALT loop,
    until cpuidle loads and either acpi_idle or intel_idle
    invoke the newer MWAIT-with-hints idle loop.

    But two machines reported problems:
    1. Certain Core2-era machines support MWAIT-C1 and HALT only.
    MWAIT-C1 is preferred for optimal power and performance.
    But if they support just C1, cpuidle never loads and
    so they use the boot-time default idle loop forever.

    2. Some laptops will boot-hang if HALT is used,
    but will boot successfully if MWAIT is used.
    This appears to be a hidden assumption in BIOS SMI,
    that is presumably valid on the proprietary OS
    where the BIOS was validated.

    https://bugzilla.kernel.org/show_bug.cgi?id=60770

    So here we effectively revert the patch above, restoring
    the mwait_idle() loop. However, we don't bother restoring
    the idle=mwait cmdline parameter, since it appears to add
    no value.

    Maintainer notes:
    For 3.9, simply revert 69fb3676df
    for 3.10, patch -F3 applies, fuzz needed due to __cpuinit use in context
    For 3.11, 3.12, 3.13, this patch applies cleanly

    Cc: Mike Galbraith <bitbucket@online.de>
    Cc: Ian Malone <ibmalone@gmail.com>
    Cc: Josh Boyer <jwboyer@redhat.com>
    Cc: <stable@vger.kernel.org> # 3.9, 3.10, 3.11, 3.12, 3.13
    Signed-off-by: Len Brown <len.brown@intel.com>
    ---
    arch/x86/kernel/process.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 46 insertions(+)

    diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
    index 3fb8d95..db471a8 100644
    --- a/arch/x86/kernel/process.c
    +++ b/arch/x86/kernel/process.c
    @@ -398,6 +398,49 @@ static void amd_e400_idle(void)
    default_idle();
    }

    +/*
    + * Intel Core2 and older machines prefer MWAIT over HALT for C1.
    + * We can't rely on cpuidle installing MWAIT, because it will not load
    + * on systems that support only C1 -- so the boot default must be MWAIT.
    + *
    + * Some AMD machines are the opposite, they depend on using HALT.
    + *
    + * So for default C1, which is used during boot until cpuidle loads,
    + * use MWAIT-C1 on Intel HW that has it, else use HALT.
    + */
    +static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
    +{
    + if (c->x86_vendor != X86_VENDOR_INTEL)
    + return 0;
    +
    + if (!cpu_has(c, X86_FEATURE_MWAIT))
    + return 0;
    +
    + return 1;
    +}
    +
    +/*
    + * MONITOR/MWAIT with no hints, used for default default C1 state.
    + * This invokes MWAIT with interrutps enabled and no flags,
    + * which is backwards compatible with the original MWAIT implementation.
    + */
    +
    +static void mwait_idle(void)
    +{
    + if (!need_resched()) {
    + if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
    + clflush((void *)&current_thread_info()->flags);
    +
    + __monitor((void *)&current_thread_info()->flags, 0, 0);
    + smp_mb();
    + if (!need_resched())
    + __sti_mwait(0, 0);
    + else
    + local_irq_enable();
    + } else
    + local_irq_enable();
    +}
    +
    void select_idle_routine(const struct cpuinfo_x86 *c)
    {
    #ifdef CONFIG_SMP
    @@ -411,6 +454,9 @@ void select_idle_routine(const struct cpuinfo_x86 *c)
    /* E400: APIC timer interrupt does not wake up CPU from C1e */
    pr_info("using AMD E400 aware idle routine\n");
    x86_idle = amd_e400_idle;
    + } else if (prefer_mwait_c1_over_halt(c)) {
    + pr_info("using mwait in idle threads\n");
    + x86_idle = mwait_idle;
    } else
    x86_idle = default_idle;
    }
    --
    1.8.5.2.309.ga25014b


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