From: Jesse Barnes <jbarnes@virtuousgeek.org>
To: george@mvista.com
Subject: Re: [RFC] (How to) Let idle CPUs sleep
Date: Thu, 12 May 2005 17:43:46 -0700
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On Thursday, May 12, 2005 3:15 pm, George Anzinger wrote:
> The timers that cause the problem are the ones that only run when the
> task is active.  These are the slice timer, the profile timer
> (ITIMER_PROF), the execution limit timer and the settime timer that
> is relative to execution time (ITIMER_VIRTUAL).

ITIMER_PROF could simply be ignored if the task it corresponds to isn't 
active when it fires, so it wouldn't incur any overhead.  
ITIMER_VIRTUAL sounds like it would uglify things though, and of course 
unused timer slice interrupts would have to be cleared out.

> Again, we can colapse all these to one, but still it needs to be
> setup when the task is switched to and removed when it is switched
> away.

Right, I see what you're saying now.  It's not as simple as I had hoped.

> Timers that run on system time (ITIMER_REAL) stay in the list even
> when the task is inactive.

Right, they'll cause the task they're associated with to become runnable 
again, or get a signal, or whatever.

> I think there is already an IPI to tell another cpu that it has work.
>  That should be enough.  Need to check, however.  From the VST point
> of view, any interrupt wake the cpu from the VST sleep, so it need
> not even target the scheduler..

But in this case you probably want it to, so it can rebalance tasks to 
the CPU that just woke up.

Jesse
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