On Thu, Jul 19, 2018 at 09:20:33AM +0200, 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? :) > > 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?
Ah, CONTEXT_TRACKING_FORCE is only for testing purpose, you should not select it, it's going to introduce overhead. Actually I should remove that. Although since we have removed CONFIG_NOHZ_FULL_ALL it's the last way we have to test NOHZ_FULL from config alone.
