On 7/31/19 9:48 AM, Rik van Riel wrote: > On Tue, 2019-07-30 at 17:01 -0400, Waiman Long wrote: >> 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. > Is that something we could fix from the TLB flushing > code? > > When switching to init_mm, drop the refcount on the > lazy mm? > > That way that overhead is not added to the context > switching code.
I have thought about that. That will require changing the active_mm of the current task to point to init_mm, for example. Since TLB flush is done in interrupt context, proper coordination between interrupt and process context will require some atomic instruction which will defect the purpose. Cheers, Longman
