> 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. So I think it’s worth at least giving this a try. > > Thanks, > Nick
