On Wed, Feb 28, 2024 at 06:43:43AM -0800, Jakub Kicinski wrote: > On Tue, 27 Feb 2024 20:42:24 -0800 Paul E. McKenney wrote: > > On Tue, Feb 27, 2024 at 07:10:01PM -0800, Jakub Kicinski wrote: > > > On Tue, 27 Feb 2024 10:32:22 -0800 Paul E. McKenney wrote: > > > > The theory is that PREEMPT_RCU kernels have preemption, and get their > > > > quiescent states that way. > > > > > > But that doesn't work well enough? > > > > > > Assuming that's the case why don't we add it with the inverse ifdef > > > condition next to the cond_resched() which follows a few lines down? > > > > > > skb_defer_free_flush(sd); > > > + > > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > > > + rcu_softirq_qs(); > > > + > > > local_bh_enable(); > > > > > > if (!repoll) > > > break; > > > > > > cond_resched(); > > > } > > > > > > We won't repoll majority of the time. > > > > I am not completely clear on what you are proposing, but one complication > > is that We need preemption disabled across calls to rcu_softirq_qs() > > and we cannot have preemption disabled across calls to cond_resched(). > > I was thinking of using rcu_all_qs(), like cond_resched() does. > Not sure how it compares in terms of functionality and cost. It is probably a bit cheaper, but it does nothing for Tasks RCU. And that "_all" in the name is a holdover from when there were separate mechanisms for bh, sched, and preempt, so maybe we should change that name. > > Another complication is that although CONFIG_PREEMPT_RT kernels are > > built with CONFIG_PREEMPT_RCU, the reverse is not always the case. > > And if we are not repolling, don't we have a high probability of doing > > a voluntary context when we reach napi_thread_wait() at the beginning > > of that loop? > > Very much so, which is why adding the cost of rcu_softirq_qs() > for every NAPI run feels like an overkill. Would it be better to do the rcu_softirq_qs() only once every 1000 times or some such? Or once every HZ jiffies? Or is there a better way? Thanx, Paul
