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. -- All Rights Reversed.
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