On Sun, Aug 11, 2019 at 02:13:18PM -0700, Paul E. McKenney wrote: [snip] > This leaves NO_HZ_FULL=y&&PREEMPT=y kernels. In that case, RCU is > more aggressive about using resched_cpu() on CPUs that have not yet > reported a quiescent state for the current grace period. Just wanted to ask something - how does resched_cpu() help for this case? Consider a nohz_full CPU and a PREEMPT=y kernel. Say a single task is running in kernel mode with scheduler tick off. As we discussed, we have no help from cond_resched() (since its a PREEMPT=y kernel). Because enough time has passed (jtsq*3), we send the CPU a re-scheduling IPI. This seems not that useful. Even if we enter the scheduler due to the rescheduling flags set on that CPU, nothing will do the rcu_report_qs_rdp() or rcu_report_qs_rnp() on those CPUs, which are needed to propagate the quiescent state to the leaf node. Neither will anything to do a rcu_momentary_dyntick_idle() for that CPU. Without this, the grace period will still end up getting blocked. Could you clarify which code paths on a nohz_full CPU running PREEMPT=y kernel actually helps to end the grace period when we call resched_cpu() on it? Don't we need atleast do a rcu_momentary_dyntick_idle() from the scheduler IPI handler or from resched_cpu() for the benefit of a nohz_full CPU? Maybe I should do an experiment to see this all play out. And I need to write down everything I learnt today before I go crazy... ;-)
