linux-kernel - Re: [RFC/PATCH] PM / Sleep: Timer quiesce in freeze state

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Message-ID: <54501CAB.8040503@linux.intel.com>
Date:	Wed, 29 Oct 2014 06:46:03 +0800
From:	"Li, Aubrey" <aubrey.li@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Brown, Len" <len.brown@...el.com>,
	"alan@...ux.intel.com" <alan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	"linux-pm@...r.kernel.org >> Linux PM list" 
	<linux-pm@...r.kernel.org>
Subject: Re: [RFC/PATCH] PM / Sleep: Timer quiesce in freeze state

On 2014/10/28 16:29, Peter Zijlstra wrote:
> On Tue, Oct 28, 2014 at 12:32:16PM +0800, Li, Aubrey wrote:
>> On 2014/10/27 15:28, Peter Zijlstra wrote:
>>> On Mon, Oct 27, 2014 at 02:27:27PM +0800, Li, Aubrey wrote:
>>>>> Now I suppose the problem is with cpu_pause() which needs IPIs to
>>>>> complete? Do we really need cpuidle_pause() there?
>>>>> cpuidle_uninstall_handlers() seems like a daft thing to call just about
>>>>> there.
>>>>
>>>> Please check the log of 8651f97bd951d0bb1c10fa24e3fa3455193f3548.
>>>> Rafael should know more this question than me.
>>>
>>> That changelog explains its complete bollocks to do it here. We _want_
>>> to enter and/or remain in deep idle states.
>>
>> cpuidle_resume() will be called at the end of dpm_resume_noirq(). So we
>> still are able to enter deep idle states after resume.
> 
> cpuidle_resume is absolute crap, as is cpuidle_suspend for that matter
> -- in this case.
> 
> The only reason we needed cpuidle_suspend is because some BIOS shat its
> pants when some CPUs were in higher C states while trying to do the S3
> thing. We're not going to use S states or BIOS calls _at_all_, so no
> point in kicking CPUs out of their deep C states.

We already kicked CPUs out of their deep C states in dpm_suspend_noirq().

We pause cpuidle in dpm_suspend_noirq() and resume cpuidle in
dpm_resume_noirq(), so currently we can't enter deep c-state during S
states. That's an intention for some buggy BIOS.

However, for freeze state, there is another intention that we want
always to enter the *deepest* c-state every time we enter freeze.
So we need cpuidle_resume() to make sure we have deep cstate  in freeze.

So back to your question in another email,

> I think you can simply remove them altogether, they're nonsense.

We need them to resume cpuidle in freeze.

Thanks,
-Aubrey

> 
> Read that changelog you referred me to again.
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