On Wed, 2019-07-31 at 10:15 -0400, Waiman Long wrote: > 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. Would it be possible to work around that by scheduling a work item that drops the active_mm? After all, a work item runs in a kernel thread, so by the time the work item is run, either the kernel will still be running the mm you want to get rid of as active_mm, or it will have already gotten rid of it earlier. -- All Rights Reversed.
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