linux-kernel - Re: [PATCH] x86: Voluntary leave

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Date:	Wed, 19 Dec 2007 20:40:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Venki Pallipadi <venkatesh.pallipadi@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <lenb@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Voluntary leave_mm before entering ACPI C3


* H. Peter Anvin <hpa@...or.com> wrote:

> Ingo Molnar wrote:
>> * Venki Pallipadi <venkatesh.pallipadi@...el.com> wrote:
>>
>>> Aviod TLB flush IPIs during C3 states by voluntary leave_mm() before 
>>> entering C3.
>>>
>>> The performance impact of TLB flush on C3 should not be significant with 
>>> respect to C3 wakeup latency. Also, CPUs tend to flush TLB in hardware 
>>> while in C3 anyways.
>>>
>
> Are there any CPUs around which *don't* flush the TLB across C3?  (I 
> guess it's not guaranteed by the spec, though, and as TLBs grow larger 
> there might be incentive to keep them online.)

i dont think it's required for C3 to even turn off any portion of the 
CPU - if an interrupt arrives after the C3 sequence is initiated but 
just before dirty cachelines have been flushed then the CPU can just 
return without touching anything (such as the TLB) - right? So i dont 
think there's any implicit guarantee of TLB flushing (nor should there 
be), but in practice, a good C3 sequence would (statistically) turn off 
large portions of the CPU and hence the TLB as well.

	Ingo
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