linux-kernel - RE: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Date:	Thu, 3 Jun 2010 07:26:55 -0700
From:	"Gross, Mark" <mark.gross@...el.com>
To:	James Bottomley <James.Bottomley@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Florian Mickler <florian@...kler.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Neil Brown <neilb@...e.de>, "tytso@....edu" <tytso@....edu>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	"felipe.balbi@...ia.com" <felipe.balbi@...ia.com>
Subject: RE: [linux-pm] [PATCH 0/8] Suspend block api (version 8)



>-----Original Message-----
>From: James Bottomley [mailto:James.Bottomley@...e.de]
>Sent: Thursday, June 03, 2010 6:25 AM
>To: Alan Cox
>Cc: Gross, Mark; Florian Mickler; Arve Hjønnevåg; Neil Brown;
>tytso@....edu; Peter Zijlstra; LKML; Thomas Gleixner; Linux OMAP Mailing
>List; Linux PM; felipe.balbi@...ia.com
>Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
>
>On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote:
>> > [mtg: ] This has been a pain point for the PM_QOS implementation.  They
>change the constrain back and forth at the transaction level of the i2c
>driver.  The pm_qos code really wasn't made to deal with such hot path use,
>as each such change triggers a re-computation of what the aggregate qos
>request is.
>>
>> That should be trivial in the usual case because 99% of the time you can
>> hot path
>>
>> 	the QoS entry changing is the latest one
>> 	there have been no other changes
>> 	If it is valid I can use the cached previous aggregate I cunningly
>> 		saved in the top QoS entry when I computed the new one
>>
>> (ie most of the time from the kernel side you have a QoS stack)
>
>It's not just the list based computation: that's trivial to fix, as you
>say ... the other problem is the notifier chain, because that's blocking
>and could be long.  Could we invoke the notifier through a workqueue?
>It doesn't seem to have veto power, so it's pure notification, does it
>matter if the notice is delayed (as long as it's in order)?
[mtg: ] true.  The notifications "could be" done on as a scheduled work item
in most cases.  I think there is only one user of the notification so far 
any way.  Most pm_qos users do a pole of the current value for whatever parameter they are interested in.

--mgross

>
>> > We've had a number of attempts at fixing this, but I think the proper
>fix is to bolt a "disable C-states > x" interface into cpu_idle that
>bypases pm_qos altogether.  Or, perhaps add a new pm_qos API that does the
>equivalent operation, overriding whatever constraint is active.
>>
>> We need some of this anyway for deep power saving because there is
>> hardware which can't wake from soem states, which in turn means if that
>> device is active we need to be above the state in question.
>
>James
>

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