> On Jul 13, 2020, at 11:31 PM, Nicholas Piggin <npiggin@xxxxxxxxx> wrote: > > 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. x86 has this exact problem. At least no more than 64*8 CPUs share the cache line :) Can your share your benchmark? > > Thanks, > Nick
