linux-kernel - Re: [PATCH 1/8] PM: Opportunistic suspend support.

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Message-Id: <201005270047.19167.rjw@sisk.pl>
Date:	Thu, 27 May 2010 00:47:19 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	"Linux-pm mailing list" <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Nigel Cunningham <nigel@...onice.net>,
	Ming Lei <tom.leiming@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.

On Thursday 27 May 2010, Alan Stern wrote:
> On Wed, 26 May 2010, Rafael J. Wysocki wrote:
> 
> > > The reason is simple: When a user process initiates an opportunistic 
> > > suspend, you make it wait in an interruptible sleep until all the 
> > > kernel suspend blockers are released.  No polling.  If another user 
> > > thread decides in the meantime that it needs to block the suspend, it 
> > > sends a signal to the power manager process.
> > > 
> > > In fact, other threads should signal the power manager process whenever 
> > > they want to block or unblock suspends.  That way the power manager 
> > > process can spend all its time sleeping, without doing any polling.
> > 
> > I still see an issue here.  Namely, if the power manager is in user space and
> > it's signaled to suspend, it has to ask the kernel to do that, presumably by
> > writing something to a sysfs file.  Then, if the kernel blocks the suspend, the
> > power manager waits until the block is released.  Now, it should go back and
> > check if user space still doesn't block suspend and if so, wait until the block
> > is released and start over.  With all suspend blockers in the kernel this
> > looping behavior is avoidable.
> 
> I must be missing something.  In Arve's patch 1/8, if the system is in
> opportunistic suspend, and a wakeup event occurs but no suspend
> blockers get enabled by the handler, what causes the system to go back
> into suspend after the event is handled?  Isn't that a loop of some
> sort?

Well, yes it is.

Rafael
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