Hello, Daniel. On Wed, Jun 05, 2019 at 11:32:29AM -0400, Daniel Jordan wrote: > Sure, quoting from the last ktask post: > > A single CPU can spend an excessive amount of time in the kernel operating > on large amounts of data. Often these situations arise during initialization- > and destruction-related tasks, where the data involved scales with system size. > These long-running jobs can slow startup and shutdown of applications and the > system itself while extra CPUs sit idle. > > To ensure that applications and the kernel continue to perform well as core > counts and memory sizes increase, harness these idle CPUs to complete such jobs > more quickly. > > ktask is a generic framework for parallelizing CPU-intensive work in the > kernel. The API is generic enough to add concurrency to many different kinds > of tasks--for example, zeroing a range of pages or evicting a list of > inodes--and aims to save its clients the trouble of splitting up the work, > choosing the number of threads to use, maintaining an efficient concurrency > level, starting these threads, and load balancing the work between them. Yeah, that rings a bell. > > For memory and io, we're generally going for remote charging, where a > > kthread explicitly says who the specific io or allocation is for, > > combined with selective back-charging, where the resource is charged > > and consumed unconditionally even if that would put the usage above > > the current limits temporarily. From what I've been seeing recently, > > combination of the two give us really good control quality without > > being too invasive across the stack. > > Yes, for memory I actually use remote charging. In patch 3 the worker's > current->active_memcg field is changed to match that of the cgroup associated > with the work. I see. > > CPU doesn't have a backcharging mechanism yet and depending on the use > > case, we *might* need to put kthreads in different cgroups. However, > > such use cases might not be that abundant and there may be gotaches > > which require them to be force-executed and back-charged (e.g. fs > > compression from global reclaim). > > The CPU-intensiveness of these works is one of the reasons for actually putting > the workers through the migration path. I don't know of a way to get the > workers to respect the cpu controller (and even cpuset for that matter) without > doing that. So, I still think it'd likely be better to go back-charging route than actually putting kworkers in non-root cgroups. That's gonna be way cheaper, simpler and makes avoiding inadvertent priority inversions trivial. Thanks. -- tejun
