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

Peter Zijlstra
2010-06-03T10:05:17Z

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)

Why would the kernel change the QoS state of a task? Why not have two
interacting QoS variables, one for the task, one for the subsystem in
question, and the action depends on their relative value?

> > 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.

Right, and I can imagine that depending on the platform details and not
the device details, so we get platform hooks in the drivers, or possible
up in the generic stack because I don't think NICs actually know if
there are open connections.