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    SubjectRe: [Patch] idle governor: Avoid lock acquisition to read pm_qos before entering idle
    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.


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