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

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Date:	Thu, 03 Jun 2010 08:24:31 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Gross, Mark" <mark.gross@...el.com>,
	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)

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)?

> > 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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