On Tue, Jun 25, 2019 at 2:52 PM Dave Hansen <dave.han...@intel.com> wrote: > > On 6/12/19 11:48 PM, Nadav Amit wrote: > > cpu_tlbstate is mostly private and only the variable is_lazy is shared. > > This causes some false-sharing when TLB flushes are performed. > > Presumably, all CPUs doing TLB flushes read 'is_lazy'. Because of this, > when we write to it we have to do the cache coherency dance to get rid > of all the CPUs that might have a read-only copy. > > I would have *thought* that we only do writes when we enter or exist > lazy mode. That's partially true. We do write in enter_lazy_tlb(), but > we also *unconditionally* write in switch_mm_irqs_off(). That seems > like it might be responsible for a chunk (or even a vast majority) of > the cacheline bounces. > > Is there anything preventing us from turning the switch_mm_irqs_off() > write into: > > if (was_lazy) > this_cpu_write(cpu_tlbstate.is_lazy, false); > > ? > > I think this patch is probably still a good general idea, but I just > wonder if reducing the writes is a better way to reduce bounces.
Good catch! I'm usually pretty good about this for test_and_set()-style things, but I totally missed this obvious unnecessary write when I did this. I hereby apologize for all the cycles I wasted :) --Andy
