linux-kernel - Re: [PATCH REGRESSION FIX] x86 idle: restore mwait

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Date:	Wed, 15 Jan 2014 10:28:51 +0100
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Len Brown <lenb@...nel.org>
Cc:	x86@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>,
	Ian Malone <ibmalone@...il.com>,
	Josh Boyer <jwboyer@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH REGRESSION FIX] x86 idle: restore mwait_idle()

On Wed, 2014-01-15 at 00:37 -0500, Len Brown wrote: 
> From: Len Brown <len.brown@...el.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@...ine.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@...ine.de>
> Cc: Ian Malone <ibmalone@...il.com>
> Cc: Josh Boyer <jwboyer@...hat.com>
> Cc: <stable@...r.kernel.org> # 3.9, 3.10, 3.11, 3.12, 3.13
> Signed-off-by: Len Brown <len.brown@...el.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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