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

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Message-ID: <AANLkTikFwznonORd42piWZvBv3oqqSj9odC9Jaud8o3x@mail.gmail.com>
Date:	Wed, 26 May 2010 16:09:12 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Dmitry Torokhov <dmitry.torokhov@...il.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 Wed, May 26, 2010 at 3:12 PM, Alan Stern <stern@...land.harvard.edu> 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?
>

Yes it is a loop. I think what you are missing is that it only loops
repeatedly if the driver that aborts suspend does not use a suspend
blocker.

> And even if it isn't, so what?  What's wrong with looping behavior?

It is a significant power drain.

> Especially a loop that's as short as this one and spends almost all of
> its time sleeping.  Think how much harder it would be to write programs
> if you weren't allowed to use any loops.  :-)
>
> Alan Stern
>
>


-- 
Arve Hjønnevåg
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