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. _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
