linux-kernel - Re: [PATCH] x86, acpi, idle: Restructure the mwait idle routines

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Message-ID: <20131120102803.GO10022@twins.programming.kicks-ass.net>
Date:	Wed, 20 Nov 2013 11:28:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>, lenb@...nel.org,
	rjw@...ysocki.net, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, shaohua.li@...el.com,
	rui.zhang@...el.com, Mike Galbraith <bitbucket@...ine.de>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, hpa@...or.com
Subject: Re: [PATCH] x86, acpi, idle: Restructure the mwait idle routines

On Tue, Nov 19, 2013 at 01:06:30PM -0800, Jacob Pan wrote:
> On Tue, 19 Nov 2013 16:13:38 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Tue, Nov 19, 2013 at 03:51:43PM +0100, Peter Zijlstra wrote:
> > > That said, that drive is completely wrecked. It uses
> > > preempt_enable_no_resched() wrong too, it has uncommented barriers..
> > > 
> > > Dude, wtf are you guys smoking?
> > 
> I applied this patch on top of upstream kernel (801a760) and found out
> my machine completely failed to enter idle when nothing is running.
> turbostate shows 100% C0. ftrace shows kernel coming in and out of idle
> frequently.
> 
> Both ACPI idle and intel_idle behaves the same way. I have to do the
> following change to allow entering C-states again.
> 
> diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
> index 80014da..b51d1e1 100644
> --- a/arch/x86/include/asm/mwait.h
> +++ b/arch/x86/include/asm/mwait.h
> @@ -49,10 +49,8 @@ static inline void mwait_idle_with_hints(unsigned
> long eax, unsigned long ecx) clflush((void
> *)&current_thread_info()->flags); 
>         __monitor((void *)&current_thread_info()->flags, 0, 0);
> -       if (!current_set_polling_and_test())
> +        if (!need_resched())
>                 __mwait(eax, ecx);
> -
> -       __current_clr_polling();
>  }
>  
>  #endif /* _ASM_X86_MWAIT_H */

That doesn't make any sense; current_set_polling_and_test() returns the
same thing need_resched() does.

But you're right, intel_idle resides 100% in C0 and acpi_idle has 100%
C1 residency... most weird.

/me goes prod at it
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