Excerpts from Nicholas Piggin's message of July 14, 2020 3:04 pm: > Excerpts from Andy Lutomirski's message of July 14, 2020 4:18 am: >> >>> On Jul 13, 2020, at 9:48 AM, Nicholas Piggin <npiggin@xxxxxxxxx> wrote: >>> >>> 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. >> >> I would be cautious with benchmarking here. I would expect that the >> nasty cases may affect power consumption more than performance — the >> specific issue is IPIs hitting idle cores, and the main effects are to >> slow down exit() a bit but also to kick the idle core out of idle. >> Although, if the idle core is in a deep sleep, that IPI could be >> *very* slow. > > It will tend to be self-limiting to some degree (deeper idle cores > would tend to have less chance of IPI) but we have bigger issues on > powerpc with that, like broadcast IPIs to the mm cpumask for THP > management. Power hasn't really shown up as an issue but powerpc > CPUs may have their own requirements and issues there, shall we say. > >> So I think it’s worth at least giving this a try. > > To be clear it's not a complete solution itself. The problem is of > course that mm cpumask gives you false negatives, so the bits > won't always clean up after themselves as CPUs switch away from their > lazy tlb mms. ^^ False positives: CPU is in the mm_cpumask, but is not using the mm as a lazy tlb. So there can be bits left and never freed. If you closed the false positives, you're back to a shared mm cache line on lazy mm context switches. Thanks, Nick
