On Mon, Mar 7, 2022 at 9:04 AM 'Michal Hocko' via kernel-team <kernel-team@xxxxxxxxxxx> wrote: > > On Thu 24-02-22 17:28:19, Suren Baghdasaryan wrote: > > Sending as an RFC to confirm if this is the right direction and to > > clarify if other tasks currently executed on mm_percpu_wq should be > > also moved to kthreads. The patch seems stable in testing but I want > > to collect more performance data before submitting a non-RFC version. > > > > > > Currently drain_all_pages uses mm_percpu_wq to drain pages from pcp > > list during direct reclaim. The tasks on a workqueue can be delayed > > by other tasks in the workqueues using the same per-cpu worker pool. > > This results in sizable delays in drain_all_pages when cpus are highly > > contended. > > This is not about cpus being highly contended. It is about too much work > on the WQ context. Ack. > > > Memory management operations designed to relieve memory pressure should > > not be allowed to block by other tasks, especially if the task in direct > > reclaim has higher priority than the blocking tasks. > > Agreed here. > > > Replace the usage of mm_percpu_wq with per-cpu low priority FIFO > > kthreads to execute draining tasks. > > This looks like a natural thing to do when WQ context is not suitable > but I am not sure the additional resources is really justified. Large > machines with a lot of cpus would create a lot of kernel threads. Can we > do better than that? > > Would it be possible to have fewer workers (e.g. 1 or one per numa node) > and it would perform the work on a dedicated cpu by changing its > affinity? Or would that introduce an unacceptable overhead? Not sure but I can try implementing per-node kthreads and measure the performance of the reclaim path, comparing with the current and with per-cpu approach. > > Or would it be possible to update the existing WQ code to use rescuer > well before the WQ is completely clogged? > -- > Michal Hocko > SUSE Labs > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
