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

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Message-ID: <alpine.LFD.2.00.1006031602160.2933@localhost.localdomain>
Date:	Thu, 3 Jun 2010 16:35:12 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	James Bottomley <James.Bottomley@...e.de>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"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>,
	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, 3 Jun 2010, James Bottomley wrote:

> 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)?

It depends on the information type and for a lot of things we might
get away without notifiers. 

The only real issue is when you need to get other cores out of their
deep idle state to make a new constraint work. That's what we do with
the DMA latency notifier right now.

Thanks,

	tglx
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