linux-kernel - Re: [patch 2/2] x86: put offline CPUs into deepest mwait cstate

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Date:	Mon, 25 May 2009 08:56:46 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 2/2] x86: put offline CPUs into deepest mwait
	cstate_subcstate

On Sat, May 23, 2009 at 07:19:42AM +0800, Pallipadi, Venkatesh wrote:
> Offline CPUs can save power by going into deepest cstate, subcstate
> instead of hlt loop.
> 
> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> ---
>  arch/x86/kernel/acpi/cstate.c |   51 +++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 51 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
> index bbbe4bb..5b0988a 100644
> --- a/arch/x86/kernel/acpi/cstate.c
> +++ b/arch/x86/kernel/acpi/cstate.c
> @@ -150,6 +150,54 @@ void acpi_processor_ffh_cstate_enter(struct acpi_processor_cx *cx)
>  }
>  EXPORT_SYMBOL_GPL(acpi_processor_ffh_cstate_enter);
>  
> +static unsigned long mwait_play_dead_eax;
> +
> +static void mwait_play_dead(void)
> +{
> +	if (boot_cpu_data.x86 >= 4)
> +		wbinvd();
> +
> +	while (1) {
> +		__monitor((void *)&current_thread_info()->flags, 0, 0);
> +		smp_mb();
> +		__mwait(mwait_play_dead_eax, 0);
> +	}
> +}
CPU is dead, can current_thread_info() still be used? Maybe just monitor a never changed
address.

Looks the patch will always take the highest native C-state, is this safe, considering
BIOS usually limit C-state?

Thanks,
Shaohua
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