On Wed, Nov 20, 2019 at 05:03:13PM -0500, Phil Auld wrote: > 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. When the current CPU isn't busy enough, there is still cost for completing request remotely. Or could we change queue_work() in the following way? * We try to queue the work to the CPU on which it was submitted, but if the * CPU dies or is saturated enough it can be processed by another CPU. Can we decide in a simple or efficient way if the current CPU is saturated enough? Thanks, Ming
