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    SubjectRE: [linux-pm] [PATCH 0/8] Suspend block api (version 8)


    >-----Original Message-----
    >From: James Bottomley [mailto:James.Bottomley@suse.de]
    >Sent: Thursday, June 03, 2010 6:25 AM
    >To: Alan Cox
    >Cc: Gross, Mark; Florian Mickler; Arve Hjønnevåg; Neil Brown;
    >tytso@mit.edu; Peter Zijlstra; LKML; Thomas Gleixner; Linux OMAP Mailing
    >List; Linux PM; felipe.balbi@nokia.com
    >Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
    >
    >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)?
    [mtg: ] true. The notifications "could be" done on as a scheduled work item
    in most cases. I think there is only one user of the notification so far
    any way. Most pm_qos users do a pole of the current value for whatever parameter they are interested in.

    --mgross

    >
    >> > We've had a number of attempts at fixing this, but I think the proper
    >fix is to bolt a "disable C-states > x" interface into cpu_idle that
    >bypases pm_qos altogether. Or, perhaps add a new pm_qos API that does the
    >equivalent operation, overriding whatever constraint is active.
    >>
    >> We need some of this anyway for deep power saving because there is
    >> hardware which can't wake from soem states, which in turn means if that
    >> device is active we need to be above the state in question.
    >
    >James
    >

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