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From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Matthew Garrett <mjg59@srcf.ucam.org>
Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.
Date: Thu, 27 May 2010 00:03:58 +0200
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Cc: Peter Zijlstra <peterz@infradead.org>,
       Arve =?iso-8859-1?q?Hj=F8nnev=E5g?= <arve@android.com>,
       Alan Stern <stern@rowland.harvard.edu>,
       Dmitry Torokhov <dmitry.torokhov@gmail.com>,
       "Linux-pm mailing list" <linux-pm@lists.linux-foundation.org>,
       Kernel development list <linux-kernel@vger.kernel.org>,
       Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
       Randy Dunlap <rdunlap@xenotime.net>,
       Andrew Morton <akpm@linux-foundation.org>,
       Andi Kleen <ak@linux.intel.com>,
       Cornelia Huck <cornelia.huck@de.ibm.com>, Tejun Heo <tj@kernel.org>,
       Jesse Barnes <jbarnes@virtuousgeek.org>,
       Nigel Cunningham <nigel@tuxonice.net>, Ming Lei <tom.leiming@gmail.com>,
       Wu Fengguang <fengguang.wu@intel.com>,
       Maxim Levitsky <maximlevitsky@gmail.com>, linux-doc@vger.kernel.org,
       Greg KH <gregkh@suse.de>, tytso@mit.edu,
       James Bottomley <James.Bottomley@suse.de>
References: <Pine.LNX.4.44L0.1005251647530.1634-100000@iolanthe.rowland.org> <1274878665.27810.354.camel@twins> <20100526132051.GA1834@srcf.ucam.org>
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On Wednesday 26 May 2010, Matthew Garrett wrote:
> 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.

I still think it would cause a loop-alike behavior between the user space
power manager and the kernel PM core to happen, because the power manager
will always have to check the user space counter after a failing suspend
attempt.

Rafael