linux-kernel - Re: [tip:x86/idle] x86, hotplug: Move WBINVD back outside the play

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Message-ID: <4C97BFE7.6050202@zytor.com>
Date:	Mon, 20 Sep 2010 13:11:19 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
CC:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	asit.k.mallick@...el.com, arjan@...ux.kernel.org,
	a.p.zijlstra@...llo.hl, lenb@...nel.org, tglx@...utronix.de,
	hpa@...ux.intel.com, linux-tip-commits@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [tip:x86/idle] x86, hotplug: Move WBINVD back outside the play_dead
 loop

On 09/20/2010 11:20 AM, H. Peter Anvin wrote:
> On 09/17/2010 05:48 PM, Venkatesh Pallipadi wrote:
>>>
>>> +       wbinvd();
>>> +
>>>        while (1) {
>>> -               mb();
>>> -               wbinvd();
>>>                __monitor(&current_thread_info()->flags, 0, 0);
>>>                mb();
>>
>>
>> Just one observation. There are some CPUs with errata that need
>> clflush before monitor. So, if that CPU wakesup spuriously it may have
>> problem reentering idle. Not sure whether that will be a problem as
>> that errata also depended on read happening on the flag. May be its
>> better to do monitor (0, 0, 0).
>>
> 
> It seems the easy way to deal with that would be to just add clflush
> before monitor... it is *probably* redundant, but it should be safe to
> do. It means depending on X86_FEATURE_CLFLUSH as well as
> X86_FEATURE_MWAIT, but I don't think there is any x86 processor which
> has MWAIT and not CLFLUSH, and I highly doubt there ever will be.
> 
> Does anyone see any downside?
> 

Patch for review... anyone see any problem with it?

	-hpa


View attachment "0001-x86-hotplug-In-the-MWAIT-case-of-play_dead-CLFLUSH-t.patch" of type "text/x-patch" (2266 bytes)