On Thu, 2010-06-03 at 16:35 +0200, Thomas Gleixner wrote: > 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. But the only DMA latency notifier is cpuidle_latency_notifier. That looks callable from atomic context, so we could have two chains: one atomic and one not. The only other notifier in use is the ieee80211_max_network_latency, which uses mutexes, so does require user context. James -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
