Messages in this thread | | | Date | Thu, 10 Feb 2011 08:20:37 -0800 | From | Andi Kleen <> | Subject | Re: [Patch] idle governor: Avoid lock acquisition to read pm_qos before entering idle |
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On Wed, Feb 09, 2011 at 09:10:42PM -0800, mark gross wrote: > On Wed, Feb 09, 2011 at 05:21:04PM -0800, Tim Chen wrote: > > I noticed that before entering idle state, the menu idle governor will > > look up the current pm_qos value according to the list of qos request > > received. This look up currently needs the acquisition of a lock to go > > down a list of qos requests to find the qos value, slowing down the > > wait a second... It gets the target_value (that is an atomic variable) > humpf. I was looking at 2.6.35, where this is true. If you want to put > back a target_value why not put it back the way it was?
I don't think the goal is to make the code look like it was before, just to have clean and scalable code going forwards.
> I'm surprised by this as the last update to the pm_qos replaced the > lists with a O(1) data structure so there was no more walking of pending > requests. > > What is the profile after the patch the Plist should be only one > dereference and an if instruction slower than a cached value.
The problem with the plist is that you need to take a lock for reading the first value. The lock is a performance problem on servers. The reference doesn't matter at all.
> Does your patch remove the need for the locks because if it doesn't I > don't see how it will make much of a difference?
The value itself doesn't need a lock, just the list access.
> > > Perhaps a better approach will be to cache the updated pm_qos value so > > reading it does not require lock acquisition as in the patch below. > > See v2.6.35 for an possible instance of the better approach.
Tim's new code seems simpler and cleaner than what was in .35.
-Andi
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