* Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote: > * Ingo Molnar <mingo@xxxxxxx> [2009-02-23 10:11:58]: > > > > > * Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote: > > > > > * Ingo Molnar <mingo@xxxxxxx> [2009-02-20 22:53:18]: > > > > > > > > > > > * Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote: > > > > > > > > > On Fri, 20 Feb 2009 17:07:37 +0100 > > > > > Ingo Molnar <mingo@xxxxxxx> wrote: > > > > > > > > > > > > > > > > > * Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > I'd also suggest to not do that rather ugly > > > > > > > > enable_timer_migration per-cpu variable, but simply reuse > > > > > > > > the existing nohz.load_balancer as a target CPU. > > > > > > > > > > > > > > This is a good idea to automatically bias the timers. But > > > > > > > this nohz.load_balancer is a very fast moving target and we > > > > > > > will need some heuristics to estimate overall system idleness > > > > > > > before moving the timers. > > > > > > > > > > > > > > I would agree that the power saving load balancer has a good > > > > > > > view of the system and can potentially guide the timer biasing > > > > > > > framework. > > > > > > > > > > > > Yeah, it's a fast moving target, but it already concentrates > > > > > > the load somewhat. > > > > > > > > > > > > > > > > I wonder if the real answer for this isn't to have timers be > > > > > considered schedulable-entities and have the regular scheduler > > > > > decide where they actually run. > > > > > > > > hm, not sure - it's a bit heavy for that. > > > > > > > > > > I think the basic timer migration policy should exist in user > > > space. > > > > I disagree. > > > > See below > > > > One of the ways of looking at it is, as we begin to > > > consolidate, using range timers and migrating all timers to > > > lesser number of CPUs would make a whole lot of sense. > > > > > > As far as the scheduler making those decisions is concerned, > > > my concern is that the load balancing is a continuous process > > > and timers don't necessarily work that way. I'd put my neck > > > out and say that irqbalance, range timers and timer migration > > > should all belong to user space. irqbalance and range timers > > > do, so should timer migration. > > > > As i said it my first reply, IRQ migration is special because > > they are not kernel-internal objects, they come externally so > > there's a lot of user-space enumeration, policy and other steps > > involved. Furthermore, IRQs are migrated in a 'slow' fashion. > > > > Timers on the other hand are fast entities tied to _tasks_ > > primarily, not external entities. > > Timers are also queued due to external events like interrupts > (device drivers tend to set of timers all the time). [...] That is a silly argument. Tasks are created due to 'external events' as well such as the user hitting a key. What matters, and what was my argument is the distinction whether the kernel _generates_ the event. For most IRQ events it does not, for the overwhelming majority of timers events it consciously generates timer events. Which makes them all the much different. > [...] I am not fully against what you've said, at some > semantic level what you are suggesting is that at a higher > level of power saving, when the scheduler balances timers it > is doing a form of soft CPU hotplug on the system by migrating > timers and tasks away from idle CPUs when the load can be > handled by other CPUs. See below as well. > > > Hence they should migrate > > according to the CPU where the activities of the system > > concentrates - i.e. where tasks are running. > > > > Another thing: do you argue for the existing timer-migration > > code we have in mod_timer() to move to user-space too? It isnt a > > consistent argument to push 'some' of it to user-space, and some > > of it in kernel-space. > > > > No.. mod_timer() is correct where it belongs. You did not reply to my statement that the argument is a double standard. Why do certain migrations in the kernel and some not? > Consider the powertop usage scenario today > > 1. Powertop displays a list of timers and common causes of wakeup > 2. It recommends policies in user space that can affect power savings > a. usb autosuspend > b. wireless link management > c. disable HAL polling That's different - those are PowerTop timer event _reduction_ policies. Not migration policies of existing timers. > My argument is, why can't we add > > d. Use range timers > e. Consolidate timers > > In the future. > > Even sched_mc=n is set by user space, so really the > policy is in user space. that is different again. sched_mc is a broad switch not a dynamic control like the sysfs migration interface that was introduced in this patchset. Which patchset we are discussing. Ingo _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
