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    SubjectRe: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
    On Thu, 3 Jun 2010, James Bottomley wrote:

    > On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote:
    > > > [mtg: ] This has been a pain point for the PM_QOS implementation. They change the constrain back and forth at the transaction level of the i2c driver. The pm_qos code really wasn't made to deal with such hot path use, as each such change triggers a re-computation of what the aggregate qos request is.
    > >
    > > That should be trivial in the usual case because 99% of the time you can
    > > hot path
    > >
    > > the QoS entry changing is the latest one
    > > there have been no other changes
    > > If it is valid I can use the cached previous aggregate I cunningly
    > > saved in the top QoS entry when I computed the new one
    > >
    > > (ie most of the time from the kernel side you have a QoS stack)
    >
    > It's not just the list based computation: that's trivial to fix, as you
    > say ... the other problem is the notifier chain, because that's blocking
    > and could be long. Could we invoke the notifier through a workqueue?
    > It doesn't seem to have veto power, so it's pure notification, does it
    > matter if the notice is delayed (as long as it's in order)?

    It depends on the information type and for a lot of things we might
    get away without notifiers.

    The only real issue is when you need to get other cores out of their
    deep idle state to make a new constraint work. That's what we do with
    the DMA latency notifier right now.

    Thanks,

    tglx


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