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

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Date:	Wed, 26 May 2010 23:51:59 +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 Wednesday 26 May 2010, Alan Stern wrote:
> On Tue, 25 May 2010, Dmitry Torokhov wrote:
> 
> > > How is that not polling? If the user is holding down a key, the keypad
> > > driver has to block suspend, and user space will try to suspend again
> > > and again and again...
> > > 
> > 
> > If your userpsace is that stupid - sure. However, you can:
> > 
> > 1. Notify the suspend manager process that he rest of your userspace is
> > busy handling keystrokes so that it does not try to suspend while there
> > are events pending.
> > 
> > 2. Wait a tiny bit after last application notified you that it finished
> > processing events.
> 
> This is more complicated than necessary.  Arve is wrong; my suggested 
> design does not require polling.
> 
> 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.

> > So basically the difference is that with in-kernel suspend blockers,
> > there is a tiny window where we haven't started the suspend yet but are
> > about to the driver has a chance to prevent entire system from starting
> > sleep.
> > 
> > Without the blocker we may start suspending and will stop midcycle. We
> > may be even better off in the end since we could leave some devices
> > still powered down after aborting system-wide suspend.
> 
> That's not so easy.  The design of the PM core requires that when the 
> system wakes up from suspend (or from an aborted suspend attempt), all 
> devices should go to full-power-on.  This is explained in section 6 of
> Documentation/power/runtime_pm.txt.
> 
> The decision over whether to use kernel-space suspend blockers vs. 
> aborting suspends when queues are non-empty comes down to a tradeoff:
> 
> 	Suspend blockers involve a little more code (to create,
> 	destroy, enable, and disable them) plus a little extra
> 	runtime overhead each time an event occurs.  (Some people
> 	may object to this extra overhead, although so far nobody
> 	in this discussion has done so.)
> 
> 	Suspend blockers eliminate the extra overhead involved in
> 	starting suspends that are just going to be aborted.
> 
> 	Suspend blockers offer extra debugging and accountability
> 	(the user can find out which subsystems are responsible for
> 	keeping the system awake).
> 
> Which way you think the tradeoff should go is a subjective decision.  I 
> think that kernel suspend blockers are acceptable.

I agree.

Rafael
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