Hi Peter, On Wed, Nov 20, 2019 at 08:16:36PM +0100 Peter Zijlstra wrote: > On Tue, Nov 19, 2019 at 07:40:54AM +1100, Dave Chinner wrote: > > On Mon, Nov 18, 2019 at 10:21:21AM +0100, Peter Zijlstra wrote: > > > > We typically only fall back to the active balancer when there is > > > (persistent) imbalance and we fail to migrate anything else (of > > > substance). > > > > > > The tuning mentioned has the effect of less frequent scheduling, IOW, > > > leaving (short) tasks on the runqueue longer. This obviously means the > > > load-balancer will have a bigger chance of seeing them. > > > > > > Now; it's been a while since I looked at the workqueue code but one > > > possible explanation would be if the kworker that picks up the work item > > > is pinned. That would make it runnable but not migratable, the exact > > > situation in which we'll end up shooting the current task with active > > > balance. > > > > Yes, that's precisely the problem - work is queued, by default, on a > > specific CPU and it will wait for a kworker that is pinned to that > > I'm thinking the problem is that it doesn't wait. If it went and waited > for it, active balance wouldn't be needed, that only works on active > tasks. Since this is AIO I wonder if it should queue_work on a nearby cpu by default instead of unbound. > > > specific CPU to dispatch it. We've already tested that queuing on a > > different CPU (via queue_work_on()) makes the problem largely go > > away as the work is not longer queued behind the long running fio > > task. > > > > This, however, is not at viable solution to the problem. The pattern > > of a long running process queuing small pieces of individual work > > for processing in a separate context is pretty common... > > Right, but you're putting the scheduler in a bind. By overloading the > CPU and only allowing the one task to migrate, it pretty much has no > choice left. > > Anyway, I'm still going to have try and reproduce -- I got side-tracked > into a crashing bug, I'll hopefully get back to this tomorrow. Lastly, > one other thing to try is -next. Vincent reworked the load-balancer > quite a bit. > I've tried it with the lb patch series. I get basically the same results. With the high granularity settings I get 3700 migrations for the 30 second run at 4k. Of those about 3200 are active balance on stock 5.4-rc7. With the lb patches it's 3500 and 3000, a slight drop. Using the default granularity settings 50 and 22 for stock and 250 and 25. So a few more total migrations with the lb patches but about the same active. On this system I'm getting 100k migrations using 512 byte blocksize. Almost all not active. I haven't looked into that closely yet but it's like 3000 per second looking like this: ... 64.19641 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.19694 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.19746 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.19665 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.19718 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.19772 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.19800 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.19828 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.19856 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.19882 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.19909 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.19937 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.19967 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.19995 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.20023 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.20053 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.20079 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.20107 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.20135 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.20163 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 64.20192 386 386 kworker/15:1 sched_migrate_task fio/2784 cpu 15->19 64.20221 389 389 kworker/19:1 sched_migrate_task fio/2784 cpu 19->15 ... Which is roughly equal to the number if iops it's doing. Cheers, Phil --
