linux-kernel - Re: suspend blockers & Android integration

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Date:	Fri, 4 Jun 2010 10:50:26 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ingo Molnar <mingo@...e.hu>
cc:	tytso@....edu, Brian Swetland <swetland@...gle.com>,
	Neil Brown <neilb@...e.de>, Arve Hj?nnev?g <arve@...roid.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Felipe Balbi <felipe.balbi@...ia.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	James Bottomley <James.Bottomley@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kevin Hilman <khilman@...prootsystems.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: suspend blockers & Android integration

On Fri, 4 Jun 2010, Ingo Molnar wrote:

> Note that this does not necessarily have to be implemented as 'execute suspend 
> from the idle task' code: scheduling from the idle task, while can certainly 
> be made to work, is a somewhat recursive concept that we might want to avoid 
> for robustness reasons.
> 
> Instead, the 'deepest idle' (suspend) method could consist of a wakeup of a 
> kernel thread (or of any of the existing kernel threads such as the migration 
> thread) - which kernel thread then does a race-free suspend: it offlines all 
> but one CPU [on platforms that need that] and then initiates the suspend - but 
> aborts the attempt if there's any sign of wakeup activity.

Out of morbid curiosity...  A typical sign of wakeup activity is a
thread becoming runnable because of expiration of a kernel timer or an
I/O completion interrupt.  How would the "race-free suspend" thread
detect this sort of thing?  Indeed, isn't the inability to detect these 
part of what makes the existing suspend implementation (the freezer in 
particular) not race-free?

Alan Stern

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