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Date: Wed, 26 May 2010 14:20:51 +0100
From: Matthew Garrett <mjg59@srcf.ucam.org>
To: Peter Zijlstra <peterz@infradead.org>
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Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.
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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@srcf.ucam.org