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: > > > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > > > > + rcu_softirq_qs(); > > > > + > > > > local_bh_enable(); > > > > > > > > if (!repoll) > > > > > > Hmm.... > > > Why napi_busy_loop() does not have a similar problem ? > > > > > > It is unclear why rcu_all_qs() in __cond_resched() is guarded by > > > > > > #ifndef CONFIG_PREEMPT_RCU > > > rcu_all_qs(); > > > #endif > > > > 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(). 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? All in all, I suspect that I am missing your point. Thanx, Paul
