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

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Message-ID: <20100526132051.GA1834@srcf.ucam.org>
Date:	Wed, 26 May 2010 14:20:51 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	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, Greg KH <gregkh@...e.de>, tytso@....edu,
	James Bottomley <James.Bottomley@...e.de>
Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.

On Wed, May 26, 2010 at 02:57:45PM +0200, Peter Zijlstra wrote:

> I fail to see why. In both cases the woken userspace will contact a
> central governing task, either the kernel or the userspace suspend
> manager, and inform it there is work to be done, and please don't
> suspend now.

Thinking about this, you're right - we don't have to wait, but that does 
result in another problem. Imagine we get two wakeup events 
approximately simultaneously. In the kernel-level universe the kernel 
knows when both have been handled. In the user-level universe, we may 
have one task schedule, bump the count, handle the event, drop the count 
and then we attempt a suspend again because the second event handler 
hasn't had an opportunity to run yet. We'll then attempt a suspend and 
immediately bounce back up. That's kind of wasteful, although it'd be 
somewhat mitigated by checking that right at the top of suspend entry 
and returning -EAGAIN or similar.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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