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

James Bottomley
2010-06-03T13:24:31Z

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)?

> > 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