On Wed, 28 Feb 2024 07:15:42 -0800 Paul E. McKenney wrote:

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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?

Right, we can do that. Yan Zhai, have you measured the performance
impact / time spent in the call?