On Fri, Aug 09, 2019 at 08:40:27PM -0700, Paul E. McKenney wrote: [snip] > > > In contrast, a heavy duty userspace-driven workload would transition to > > > and from userspace for each kfree_rcu(), and that would increment the > > > dyntick-idle count on each transition to and from userspace. Adding the > > > rcu_momentary_dyntick_idle() emulates a pair of such transitions. > > > > But even if we're in kernel mode and not transitioning, I thought the FQS > > loop (rcu_implicit_dynticks_qs() function) would set need_heavy_qs to true at > > 2 * jiffies_to_sched_qs. > > > > Hmm, I forgot that jiffies_to_sched_qs can be quite large I guess. You're > > right, we could call rcu_momentary_dyntick_idle() in advance before waiting > > for FQS loop to do the setting of need_heavy_qs. > > > > Or, am I missing something with the rcu_momentary_dyntick_idle() point you > > made? > > The trick is that rcu_momentary_dyntick_idle() directly increments the > CPU's dyntick counter, so that the next FQS loop will note that the CPU > passed through a quiescent state. No need for need_heavy_qs in this case. Yes, that's what I also understand. Thanks for confirming, - Joel
