On 7/29/19 8:26 PM, Rik van Riel wrote: > On Mon, 2019-07-29 at 17:42 -0400, Waiman Long wrote: > >> What I have found is that a long running process on a mostly idle >> system >> with many CPUs is likely to cycle through a lot of the CPUs during >> its >> lifetime and leave behind its mm in the active_mm of those CPUs. My >> 2-socket test system have 96 logical CPUs. After running the test >> program for a minute or so, it leaves behind its mm in about half of >> the >> CPUs with a mm_count of 45 after exit. So the dying mm will stay >> until >> all those 45 CPUs get new user tasks to run. > OK. On what kernel are you seeing this? > > On current upstream, the code in native_flush_tlb_others() > will send a TLB flush to every CPU in mm_cpumask() if page > table pages have been freed. > > That should cause the lazy TLB CPUs to switch to init_mm > when the exit->zap_page_range path gets to the point where > it frees page tables. > I was using the latest upstream 5.3-rc2 kernel. It may be the case that the mm has been switched, but the mm_count field of the active_mm of the kthread is not being decremented until a user task runs on a CPU. >>> If it is only on the CPU where the task is exiting, >>> would the TASK_DEAD handling in finish_task_switch() >>> be a better place to handle this? >> I need to switch the mm off the dying one. mm switching is only done >> in >> context_switch(). I don't think finish_task_switch() is the right >> place. > mm switching is also done in flush_tlb_func_common, > if the CPU received a TLB shootdown IPI while in lazy > TLB mode. > I see. Cheers, Longman
