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SubjectRe: [PATCH REGRESSION FIX] x86 idle: restore mwait_idle()
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On Wed, 2014-01-15 at 00:37 -0500, Len Brown wrote: 
> 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.

Q6600 box (allegedly) has a slightly greenish tinge again.

Tested-by: Mike Galbraith <bitbucket@online.de>

> 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;
> }




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