On Mon, 2019-07-29 at 11:37 -0400, Waiman Long wrote: > On 7/29/19 11:03 AM, Peter Zijlstra wrote: > > On Mon, Jul 29, 2019 at 10:51:51AM -0400, Waiman Long wrote: > > > On 7/29/19 4:52 AM, Peter Zijlstra wrote: > > > > On Sat, Jul 27, 2019 at 01:10:47PM -0400, Waiman Long wrote: > > > > > It was found that a dying mm_struct where the owning task has > > > > > exited > > > > > can stay on as active_mm of kernel threads as long as no > > > > > other user > > > > > tasks run on those CPUs that use it as active_mm. This > > > > > prolongs the > > > > > life time of dying mm holding up memory and other resources > > > > > like swap > > > > > space that cannot be freed. > > > > Sure, but this has been so 'forever', why is it a problem now? > > > I ran into this probem when running a test program that keeps on > > > allocating and touch memory and it eventually fails as the swap > > > space is > > > full. After the failure, I could not rerun the test program again > > > because the swap space remained full. I finally track it down to > > > the > > > fact that the mm stayed on as active_mm of kernel threads. I have > > > to > > > make sure that all the idle cpus get a user task to run to bump > > > the > > > dying mm off the active_mm of those cpus, but this is just a > > > workaround, > > > not a solution to this problem. > > The 'sad' part is that x86 already switches to init_mm on idle and > > we > > only keep the active_mm around for 'stupid'. > > > > Rik and Andy were working on getting that 'fixed' a while ago, not > > sure > > where that went. > > Good, perhaps the right thing to do is for the idle->kernel case to > keep > init_mm as the active_mm instead of reuse whatever left behind the > last > time around. Absolutely not. That creates heavy cache line contention on the mm_cpumask as we switch the mm out and back in after an idle period. The cache line contention on the mm_cpumask alone can take up as much as a percent of CPU time on a very busy system with a large multi-threaded application, multiple sockets, and lots of context switches. -- All Rights Reversed.
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