Re: OnDemand governor vs. work queue

[Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>]

On Thu, May 26, 2005 at 07:48:27AM +1000, Benjamin Herrenschmidt wrote:
> 
> > Agreed. For the larger latency transitions, ondemand governor should 
> > use its own thread rather than keventd. Probably, it may make sense to 
> > have separate threads for each CPU as well in that case (as otherwise 
> > this thread will end up being moved around from one CPU to other. I will
> > 
> > look at this in a couple of weeks unless someone beats me to it and 
> > sends the patch before that :).
> 
> A per-CPU thread could be a custom work queue (easy to implement form
> current code) but also a resource waste. I don't think there is that
> much of a problem having that thread moved around.
> 

This is how things work today.
- ondemand (keventd) wakes up on some CPU A.
- Then we loop through all the CPU Is
	- Figure out whether we need to increase the frequency
		- If yes, the underlying governor may (on i386, x86-64 it will)
		  move the thread to CPU I
		- Change the frequency
		- Move the thread back to CPU A.


And each of this affinity calls will wakeup migration thread to do the proper 
affinity and also IPIs for resched. At this moment I am not sure how much 
overhead we have due to this affinity. I need to look closely whether this 
is overhead is significant (Especially on systems with 4 or more CPUs).

-Thanks,
Venki