On Wed, Apr 19, 2017 at 01:48:08PM +0200, Christian Borntraeger wrote: > On 04/19/2017 01:28 PM, Peter Zijlstra wrote: > > > > So the thing Maz complained about is because KVM assumes > > synchronize_srcu() is 'free' when there is no srcu_read_lock() activity. > > This series 'breaks' that. > > Why is such a behaviour change not mentioned in the cover letter? > I could not find anything in the patch descriptions that would > indicate a slowdown. How much slower did it get? > > But indeed, there are several places at KVM startup which have been > reworked to srcu since normal rcu was too slow for several usecases. > (Mostly registering devices and related data structures at startup, > basically the qemu/kvm coldplug interaction)
I suspect Paul is not considering this a 'normal' RCU feature, and therefore didn't think about changing this. I know I was fairly surprised by this requirement when I ran into it; and only accidentally remembered it now that maz complained.
