Hi, On Thu, Jul 09, 2015 at 03:32:20PM +0100, Morten Rasmussen wrote: > On Mon, Jul 06, 2015 at 06:31:44AM +0800, Yuyang Du wrote: > > On Fri, Jul 03, 2015 at 06:38:31PM +0200, Peter Zijlstra wrote: > > > > I'm not against having a policy that sits somewhere in between, we just > > > > have to agree it is the right policy and clean up the load-balance code > > > > such that the implemented policy is clear. > > > > > > Right, for balancing its a tricky question, but mixing them without > > > intent is, as you say, a bit of a mess. > > > > > > So clearly blocked load doesn't make sense for (new)idle balancing. OTOH > > > it does make some sense for the regular periodic balancing, because > > > there we really do care mostly about the averages, esp. so when we're > > > overloaded -- but there are issues there too. > > > > > > Now we can't track them both (or rather we could, but overhead). > > > > > > I like Yuyang's load tracking rewrite, but it changes exactly this part, > > > and I'm not sure I understand the full ramifications of that yet. > > I don't think anybody does ;-) But I think we should try to make it > work. > > > Thanks. It would be a pure average policy, which is non-perfect like now, > > and certainly needs a mixing like now, but it is worth a starter, because > > it is simple and reasaonble, and based on it, the other parts can be simple > > and reasonable. > > I think we all agree on the benefits of taking blocked load into > account but also that there are some policy questions to be addressed. > > > > One way out would be to split the load balancer into 3 distinct regions; > > > > > > 1) get a task on every CPU, screw everything else. > > > 2) get each CPU fully utilized, still ignoring 'load' > > > 3) when everybody is fully utilized, consider load. > > Seems very reasonable to me. We more or less follow that idea in the > energy-model driven scheduling patches, at least 2) and 3). > > The difficult bit is detecting when to transition between 2) and 3). If > you want to enforce smp_nice you have to start worrying about task > priority as soon as one cpu is fully utilized. > > For example, a fully utilized cpu has two high priority tasks while all > other cpus are running low priority tasks and are not fully utilized. > The utilization imbalance may be too small to cause any tasks to be > migrated, so we end up giving fewer cycles to the high priority tasks. > > > > If we make find_busiest_foo() select one of these 3, and make > > > calculate_imbalance() invariant to the metric passed in, and have things > > > like cpu_load() and task_load() return different, but coherent, numbers > > > depending on which region we're in, this almost sounds 'simple'. > > > > > > The devil is in the details, and the balancer is a hairy nest of details > > > which will make the above non-trivial. > > Yes, but if we have an overall policy like the one you propose we can at > least make it complicated and claim that we think we know what it is > supposed to do ;-) > > I agree that there is some work to be done in find_busiest_*() and > calcuate_imbalance() + friends. Maybe step one should be to clean them > up a bit.
Consensus looks like that we move step-by-step and start working right now: 1) Based on the "Rewrite" patch, let me add cfs_rq->runnable_load_avg. Then we will have up-to-date everything: load.weight, runnable_load_avg, and load_avg (including runnable + blocked), from pure now to pure average. The runnable_load_avg will be used the same as now. So we will not have a shred of remification. As long as the code is cleared and simplified, it is a win. 2) Let's clean up a bit the load balancing part code-wise, and if needed, make change to the obvious things, otherwise leave it unchanged. 3) Polish/complicate the policies, :) What do you think? Thanks, Yuyang -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
