On Sun, May 11, 2014 at 03:30:31AM +0200, Frederic Weisbecker wrote: > On Sat, May 10, 2014 at 06:17:08PM -0700, Paul E. McKenney wrote: > > On Sat, May 10, 2014 at 03:14:25PM +0200, Frederic Weisbecker wrote: > > > On Sat, May 10, 2014 at 02:31:28PM +0200, Thomas Gleixner wrote: > > > > On Sat, 10 May 2014, Frederic Weisbecker wrote: > > > > > But I still have the plan to make the timekeeper use the full sysidle > > > > > facility in order to adaptively get to dynticks idle. > > > > > > > > > > Reminder for others: in NO_HZ_FULL, the timekeeper (always CPU 0) stays > > > > > completely periodic. It can't enter in dynticks idle mode because it > > > > > must maintain timekeeping on behalf of full dynticks CPUs. So that's > > > > > a power issue. > > > > > > > > > > But Paul has a feature in RCU that lets us know when all CPUs are idle > > > > > and the timekeeper can finally sleep. Then when a full nohz CPU wakes > > > > > up from idle, it sends an IPI to the timekeeper if needed so the latter > > > > > restarts timekeeping maintainance. > > > > > > > > > > It's not complicated to add to the timer code. > > > > > Most of the code is already there, in RCU, for a while already. > > > > > > > > > > Are we keeping that direction? > > > > > > > > So the idea is that the timekeeper stays on cpu0, but if everything is > > > > idle it is allowed to take a long nap as well. So if some other cpu > > > > wakes up it updates timekeeping without taking over the time keeper > > > > duty and if it has work to do, it kicks cpu0 into gear. If it just > > > > goes back to sleep, then nothing to do. > > > > Hmmm... If RCU is supposed to ignore the fact that one of the other > > CPUs woke up momentarily, we will need to adjust things a bit. > > Maybe not that much actually. > > > > > > Exactly! Except perhaps the last sentence "If it just goes back to sleep, > > > then nothing to do.", I didn't think about that although this special case > > > is quite frequent indeed when an interrupt fires on idle but no task is woken up. > > > > > > Maybe I should move the code that fires the IPI to cpu0, if it is sleeping, > > > on irq exit (the plan was to do it right away on irq enter) and fire it > > > only if need_resched(). > > > > And of course if that code path contains any RCU read-side critical > > sections, RCU absolutely cannot ignore that CPU's momentary wakeup. > > Sure the core RCU still needs to know that the CPU went out of dynticks the > time of the irq, so we keep the rcu_irq_enter/rcu_irq_exit calls. > > But if the CPU only wakes up to serve an IRQ, it doesn't need to tell the RCU > sysidle detection about it. The irq entry fixup jiffies on dynticks idle mode, > this should be enough. As long as you pass me in a hint so that RCU knows which case it is dealing with. ;-) Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>