On Thu, Jul 19, 2018 at 12:23:34PM +0200, Christian Borntraeger wrote: > > > On 07/19/2018 09:20 AM, David Woodhouse wrote: > > On Thu, 2018-07-19 at 08:45 +0200, Christian Borntraeger wrote: > >> > >>> My thought would be something like this: > >>> > >>> if (context_tracking_cpu_is_enabled()) > >>> rcu_kvm_enter(); > >>> else > >>> rcu_virt_note_context_switch(smp_processor_id()); > >> > >> In the past we needed that (when we introduced that). At least with every > >> host interrupt we called this making an rcu event at least every HZ. > >> Will the changes in need_resched make this part unnecessary? > > > > Yes, the change in need_resched() should make this part unnecessary. > > Unless your architecture's version of the vcpu_run() loop just loops > > forever even when TIF_NEED_RESCHED is set? :) > > Very early versions did that. The SIE instruction is interruptible > so you can continue to run the guest by simply returning from an host > interrupt. All sane versions of KVM on s390 now make sure to make a > short trip into C after a host interrupt. There we check for > need_resched signals and machine checks so we are good. OK, thank you all! I will drop the two patches that add the rcu_kvm_enter() and rcu_kvm_exit() calls. Two less things to worry about! ;-) Thanx, Paul > > I'm not sure about the context tracking condition in the code snippet > > cited above, though. I think that's what caused my problem in the first > > place — I have CONTEXT_TRACKING_FORCE && !NO_HZ_FULL. So in 4.15, that > > means rcu_user_enter() did nothing and rcu_virt_note_context_switch() > > wasn't called. Hence the observed stalls. > > > > Should rcu_user_enter() itself be conditional on CONTEXT_TRACKING not > > NO_HZ_FULL? > >
