Excerpts from Andy Lutomirski's message of July 14, 2020 1:59 am: > 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. I don't think it's nonsense, it could be a good way to avoid IPIs. I haven't seen much problem here that made me too concerned about IPIs yet, so I think the simple patch may be good enough to start with for powerpc. I'm looking at avoiding/reducing the IPIs by combining the unlazying with the exit TLB flush without doing anything fancy with ref counting, but we'll see. Thanks, Nick
