On Thu, Jul 9, 2020 at 6:57 PM Nicholas Piggin <npiggin@xxxxxxxxx> wrote: > > On big systems, the mm refcount can become highly contented when doing > a lot of context switching with threaded applications (particularly > switching between the idle thread and an application thread). > > Abandoning lazy tlb slows switching down quite a bit in the important > user->idle->user cases, so so instead implement a non-refcounted scheme > that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down > any remaining lazy ones. > > On a 16-socket 192-core POWER8 system, a context switching benchmark > with as many software threads as CPUs (so each switch will go in and > out of idle), upstream can achieve a rate of about 1 million context > switches per second. After this patch it goes up to 118 million. > I read the patch a couple of times, and I have a suggestion that could be nonsense. You are, effectively, using mm_cpumask() as a sort of refcount. You're saying "hey, this mm has no more references, but it still has nonempty mm_cpumask(), so let's send an IPI and shoot down those references too." I'm wondering whether you actually need the IPI. What if, instead, you actually treated mm_cpumask as a refcount for real? Roughly, in __mmdrop(), you would only free the page tables if mm_cpumask() is empty. And, in the code that removes a CPU from mm_cpumask(), you would check if mm_users == 0 and, if so, check if you just removed the last bit from mm_cpumask and potentially free the mm. Getting the locking right here could be a bit tricky -- you need to avoid two CPUs simultaneously exiting lazy TLB and thinking they should free the mm, and you also need to avoid an mm with mm_users hitting zero concurrently with the last remote CPU using it lazily exiting lazy TLB. Perhaps this could be resolved by having mm_count == 1 mean "mm_cpumask() is might contain bits and, if so, it owns the mm" and mm_count == 0 meaning "now it's dead" and using some careful cmpxchg or dec_return to make sure that only one CPU frees it. Or maybe you'd need a lock or RCU for this, but the idea would be to only ever take the lock after mm_users goes to zero. --Andy
