On Tue, Dec 08, 2020 at 11:04:38PM +0100, Frederic Weisbecker wrote: > On Tue, Dec 08, 2020 at 10:24:09AM -0800, Paul E. McKenney wrote: > > > It reduces the code scope running with BH disabled. > > > Also narrowing down helps to understand what it actually protects. > > > > I thought that you would call out unnecessarily delaying other softirq > > handlers. ;-) > > > > But if such delays are a problem (and they might well be), then to > > avoid them on non-rcu_nocb CPUs would instead/also require changing the > > early-exit checks to check for other pending softirqs to the existing > > checks involving time, need_resched, and idle. At which point, entering and > > exiting BH-disabled again doesn't help, other than your point about the > > difference in BH-disabled scopes on rcu_nocb and non-rcu_nocb CPUs. > > Wise observation! > > > Would it make sense to exit rcu_do_batch() if more than some amount > > of time had elapsed and there was some non-RCU softirq pending? > > > > My guess is that the current tlimit checks in rcu_do_batch() make this > > unnecessary. > > Right and nobody has complained about it so far. If they did, my thought would be to add another early-exit check, but under the tlimit check, so that pending non-RCU softirqs might set a shorter time limit. For example, instead of allowing up to the current one second in rcu_do_batch(), allow only up to 100 milliseconds or whatever. But there are lots of choices, which is one reason to wait until it becomes a problem. > But I should add a comment explaining the reason for the BH-disabled > section in my series. That sounds like a most excellent idea, please do! Thanx, Paul
