On Sun, Aug 11, 2019 at 11:21:42PM -0400, Joel Fernandes wrote: > 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. An experiment would be good! > And I need to write down everything I learnt today before I go crazy... ;-) I know that feeling! ;-) Thanx, Paul
