> On Dec 3, 2020, at 9:09 AM, Alexander Gordeev <agordeev@xxxxxxxxxxxxx> wrote: > > On Mon, Nov 30, 2020 at 10:31:51AM -0800, Andy Lutomirski wrote: >> other arch folk: there's some background here: > >> >> power: Ridiculously complicated, seems to vary by system and kernel config. >> >> So, Nick, your unconditional IPI scheme is apparently a big >> improvement for power, and it should be an improvement and have low >> cost for x86. On arm64 and s390x it will add more IPIs on process >> exit but reduce contention on context switching depending on how lazy > > s390 does not invalidate TLBs per-CPU explicitly - we have special > instructions for that. Those in turn initiate signalling to other > CPUs, completely transparent to OS. Just to make sure I understand: this means that you broadcast flushes to all CPUs, not just a subset? > > Apart from mm_count, I am struggling to realize how the suggested > scheme could change the the contention on s390 in connection with > TLB. Could you clarify a bit here, please? I’m just talking about mm_count. Maintaining mm_count is quite expensive on some workloads. > >> TLB works. I suppose we could try it for all architectures without >> any further optimizations. Or we could try one of the perhaps >> excessively clever improvements I linked above. arm64, s390x people, >> what do you think? > > I do not immediately see anything in the series that would harm > performance on s390. > > We however use mm_cpumask to distinguish between local and global TLB > flushes. With this series it looks like mm_cpumask is *required* to > be consistent with lazy users. And that is something quite diffucult > for us to adhere (at least in the foreseeable future). You don’t actually need to maintain mm_cpumask — we could scan all CPUs instead. > > But actually keeping track of lazy users in a cpumask is something > the generic code would rather do AFAICT. The problem is that arches don’t agree on what the contents of mm_cpumask should be. Tracking a mask of exactly what the arch wants in generic code is a nontrivial operation.
