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

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Date:	Thu, 03 Jun 2010 12:05:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Gross, Mark" <mark.gross@...el.com>,
	Florian Mickler <florian@...kler.org>,
	James Bottomley <James.Bottomley@...e.de>,
	Arve Hjønnevåg <arve@...roid.com>,
	Neil Brown <neilb@...e.de>, "tytso@....edu" <tytso@....edu>,
	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)

Why would the kernel change the QoS state of a task? Why not have two
interacting QoS variables, one for the task, one for the subsystem in
question, and the action depends on their relative value?


> > 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.

Right, and I can imagine that depending on the platform details and not
the device details, so we get platform hooks in the drivers, or possible
up in the generic stack because I don't think NICs actually know if
there are open connections.
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