On Thu, 2018-07-12 at 13:00 +0200, Christian Borntraeger wrote: > > On s390 this seems to add about 10ns (~3%) for a guest exit/rentry > microbenchmark mostly due to rcu_eqs_enter and rcu_eqs_exit now being > visible in perf samples. The older interface was cheaper.
Well, the older interface wasn't actually working, which made it moderately suboptimal :) But that is fixed by the first patch of the two, so perhaps the second ought to be conditional on CONFIG_NO_HZ_FULL since it's only really fixing a real bug there? Unless we can contrive some way to do the rcu_eqs_enter/exit only on the paths to/from userspace, and *not* when we loop in the kernel to handle interrupts (etc.) and immediately re-enter the guest without returning? That's somewhat non-trivial though...
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