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

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Date:	Mon, 31 May 2010 23:38:55 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Florian Mickler <florian@...kler.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Brian Swetland <swetland@...gle.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	markgross@...gnar.org
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Monday 31 May 2010, Arve Hjønnevåg wrote:
> 2010/5/29 Alan Stern <stern@...land.harvard.edu>:
> > On Sat, 29 May 2010, Arve Hjønnevåg wrote:
> >
> >> > In place of in-kernel suspend blockers, there will be a new type of QoS
> >> > constraint -- call it QOS_EVENTUALLY.  It's a very weak constraint,
> >> > compatible with all cpuidle modes in which runnable threads are allowed
> >> > to run (which is all of them), but not compatible with suspend.
> >> >
> >> This sound just like another API rename. It will work, but given that
> >> suspend blockers was the name least objectionable last time around,
> >> I'm not sure what this would solve.
> >
> > It's not just a rename.  By changing this into a QoS constraint, we
> > make it more generally useful.  Instead of standing on its own, it
> > becomes part of the PM-QOS framework.
> >
> 
> We cannot use the existing pm-qos framework. It is not safe to call
> from atomic context.

We've just merged a patch that fixed that if I'm not mistaken.  Mark, did your
PM QoS update fix that?

> Also, it does not have any state constraints, so it iterates over every
> registered constraint each time one of them changes.

That's fixable IMO.

> Nor does is currently provide any stats for debugging.

That's why Alan is proposing to add that.

> The original wakelock patchset supported a wakelock type so it could
> be used to block more then suspend, but I had to remove this because
> it "overlapped" with pm-qos. So, yes I do consider this just another
> rename.

It's an extension of an existing framework rather than an addition of a new
one, with entirely new API and so on.  Extending existing APIs is much
preferred to adding new ones, in general.

Rafael
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