linux-kernel - RE: [PATCH] x86: Add clflush before monitor for Intel 7400 series

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Message-ID: <7E82351C108FA840AB1866AC776AEC4637B798D6@orsmsx505.amr.corp.intel.com>
Date:	Fri, 17 Oct 2008 13:05:00 -0700
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86: Add clflush before monitor for Intel 7400 series



>-----Original Message-----
>From: H. Peter Anvin [mailto:hpa@...or.com]
>Sent: Tuesday, October 07, 2008 4:03 PM
>To: Pallipadi, Venkatesh
>Cc: Ingo Molnar; Thomas Gleixner; linux-kernel
>Subject: Re: [PATCH] x86: Add clflush before monitor for Intel
>7400 series
>
>Venki Pallipadi wrote:
>
>> Yes. Other alternative is to disable mwaits altogether on
>these CPUs. I can
>> send a patch to do that. But, the patch will be somewhat
>more complicated
>> as kernel advertises the MWAIT capability to firmware with a
>ACPI _PDC method
>> and BIOS has to then give IO port based C2 for us to use in
>such case.
>> Avoiding mwait just for C1 is easy though.
>
>It would be my weak preference, although I'm willing to be convinced
>that mwait is worth the power savings even with the workaround.
>
>        -hpa

hpa,

Do you still have reservations about this being expensive.
Note that this is only done when a CPU is about to go idle and
The cost of clflush itself will be minimal compared to idle
entry + idle exit latency.

Thanks,
Venki
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